So after ACN you finally become SMN. Finally, you can become that classical final fantasy job that you always wanted to be. But, in Final Fantasy XIV, this is really not the case. SMN isn't what it should be, it isn't what it needs to be.
Now, keep in mind, this post is gonna try to avoid mathing out specific DPS numbers, values, potencies, etc. If you wanna theorycraft DPS with me, by all means, let's talk in game or somewhere else. I push SMN DPS very hard, and it's actually quite good. We can math up potencies and talk dot snapshotting with contagion and all kinds of stuff all day long, but that isn't what this is about.
Let's review the skills you get for becoming a Summoner.
Summon III - Useless
You learn how to summon Ifrit. The only use for Ifrit that possibly exists is as a temporary pet from 30 till 40. It is even arguable that Emerald Carbuncle is better than Ifrit, but it's pretty close so I won't bother.
Ifrit serves no real purpose. He has no useful skills that make him better than Garuda. Not only is Garuda's auto attack DPS higher than Ifrit (or extremely close) but Garuda is ranged, and thus more likely to survive and in a much better situation in general. Even IF Ifrit did slightly more damage than Garuda, or his skills did slightly more damage, Garuda's Contagion completely blows him away in every regard. Contagion immediately makes this a no contest, and Garuda is immediately 100% superior. Even when you hit level 40, immediately, switching over to Emerald Carbuncle is the better option because of Shining Emerald (which is the same as contagion).
Why is Contagion so good? Because it's +15 seconds to all your dots. Multiply the DoT potency of each dot by 5 (they each tick 5 more times), add them all up, and Contagion is effectively a nuke of its own that hits for that much potency. Ifrit cannot even come anywhere close to this level of effectiveness, and with synergies like Bane and other abilities... Ifrit is now immediately useless. Combine this with the fact that now you don't have to recast your dots as soon as you normally would, opening up more cast time and GCDs for ruins or anything else you might need to do, even if it's just mobility.
So what purpose does Ifrit serve? The only useful thing he has that Garuda does not is a stun. The stun does not last long enough to serve as anything other than an interrupt, lasting a measly 2 seconds. All it basically does is reset the enemy's swing timer for auto attacks, which in the end doesn't do you much good. On top of all that, it increases the enemy's resistance to stun, engaging diminishing returns; this hurts for the people that actually have GOOD stuns (paladin). The amount of DPS you lose for not having Contagion, combined with the danger of having a melee pet getting hit by many more attacks than normal, makes this not worth it at all. This is in addition to the fact that you can't order a pet to use a skill while casting (thus making interrupting with proper timing difficult) AND he won't use the stun until he's done with his current action.
How about Radiant Shield? Such a cool skill huh? Hahaha... no. Why in the world SE thought this was a good idea is beyond me. While nifty, it serves nearly no purpose. It is an extremely rare scenario that Ifrit is ever struck physically. If Ifrit is getting hit, that means you aren't hitting the mob, which means something is wrong or you're going to be in a couple seconds. He has no method of holding hate, at all, which means this is extremely rarely useful. If you're using obey on your pet like you should be, it's not even worth using for the amount of GCD/animation/whatever it takes to use it. You're going to pull it off him in a couple seconds regardless; the amount of time it takes for him to execute the skill won't come close to the damage he would've done with a normal attack.
All in all, Ifrit is useless. At level 50, any serious summoner would never, ever use him. He serves no purpose in any way besides his stun, which I've already explained why is so mediocre. In any difficult or serious content you will be using Garuda, thus making this skill useless.
Fester - Amazing
Fester is simple, effective, and amazing. It's not a skill that has ANYTHING to do with summoning, but this skill alone carries the Summoner skillset. This skill ends up being one of your main nukes, and while I'm not going to go into DPS rotation here, this is essential.
Tri-disaster - Mostly useless
Tri-disaster is mostly useless. The purpose it serves is to have a bind, and also have the ability to dot up an enemy and follow up with tri-disaster, making them tick from dots and take tons of damage while being unable to move. This works on anything melee, as long as they don't have a ranged attack. It can also be used as an AoE nuke provided there's a high enough quantity of targets, though in many situations you won't have time to use it (if there's a lot of enemies, Thunder multi-dotting ends up winning usually). It's kind of annoying to do so though due to it being one of those spells that has a cast time longer than the GCD, making it somewhat inefficient, combined with the large number of other things you COULD be doing instead.
Tri-disaster's only real usefulness comes in solo play or adds in dungeons. Most of the time the bind will break prematurely, either because Garuda attacks it or a party member does. Yes, Garuda won't attack a target that is bound, but if she is already mid cast when your bind goes off, she won't care, requiring manual intervention. Square Enix likes to state that classes are for soloing and jobs are for party play, so I'm not sure why we got a skill that is really only useful while soloing. In a party, it DOES have its moments and I do use it, but it's extremely rare, useless most of the time, usually only pops up when your party is wiping or something, and again, has absolutely nothing to do with summoning.
Spur - Good
Spur is useful. It is another damage buff, and you can't really argue with that. It's EXTREMELY similar to Rouse, which makes me wonder why in the world they would be so lazy to give us another one, but whatever, it's okay. You can stack the two up for your ultimate skill, which is the main use, besides just a DPS increase on Garuda here and there (like with her AoE). It actually has something to do with your summon this time, so I can't complain that much, but it's still pretty lazy.
Enkindle - Meh
This skill is really underwhelming. Besides the buggy nature of it (many times, depending on the situation, your pet simply won't do it and the cooldown will be wasted) it's really not that good. Using Rouse and Spur at the same time with it, it ends up being around 600-700 damage to everyone in the area of effect at level 50. While this is fairly potent (being around the same damage as Fester) it comes with an extremely long cooldown of 5 minutes.
The skill, compared to most job ultimates, is not that interesting. It doesn't change up our rotation, it doesn't offer us useful utility, it's just a nuke. In an aoe situation, you use it. Woo. Damage. That's it. It's not a very cool effect, and in single target boss fights, it's just another Fester. That's not to say it's bad; it does do damage, but... it's really not that good. It's a decent amount of burst damage when you need it, but it doesn't even nearly match up to Astral Flow, or the usefulness of a lot of other job 50 skills. You press it, and the pet does a good attack. There's no thinking here or anything, you just use it when it's up with Rouse and Spur. It's not interesting, it doesn't feel fun, and the worst part of it all is when it bugs and the pet does nothing at all. The damage isn't big enough to have a "wow" factor, and while it is an AoE (and thus pretty potent for an AoE) the cooldown is far too long to have any real significance. I don't mind the skill, but as our final skill, it's pretty depressing, especially when it's on a cooldown of FIVE MINUTES.
Okay, we also have to keep in mind that playing Summoner allows you to use Garuda instead of Emerald Carbuncle, which is a slight DPS increase for her. They have the exact same skills, but Garuda does do a little bit more damage, so there's that.
So, all in all, what useful things do we get for becoming Summoner?
-Slight pet damage increase
-Fester
-Spur
-Enkindle
The slight pet damage increase is "shrug", it's nice, but it's not interesting to be called a summoner mechanic. Fester is amazing and probably the only saving grace of the soul crystal, being a crucial part of your rotation, even though it has nothing to do with summoning. The majority of the time you will want to save Spur and use it with Enkindle depending on the situation, and Enkindle is a nice skill, but it's nice EVERY FIVE MINUTES. It's not very "interesting", as I've already described, and doesn't have a big "wow" factor. It's just an extra damage button.
Seriously Square Enix? This is what we get for becoming a SUMMONER? All in all, we essentially learn two nukes and our pet damage goes up slightly (even if you say "Spur is useful other than for Enkindle", it's still just a pet damage increase in the long run). This doesn't make me feel like a summoner at all; you just basically continue to play ACN, and your damage went up slightly from your pet. Now you have a good nuke to use instead of energy drain, but in any level 50 content, all you're doing is playing ACN. There is no synergy or gameplay with your pet at all. If there's 2+ mobs you'll press Aerial Slash, and you'll press Contagion when you need it, but that's it. She'll sit there auto attacking while you do your own rotation. The gameplay is minimally focused on the pet, and becoming a Summoner does NOTHING to change that. You would still use Downburst and Shining Emerald even if you weren't a Summoner. Becoming a summoner just reminds you to press that Spur button when you press Rouse and Enkindle. Don't get me wrong, the class/job isn't bad; if played properly, it's effective, does good DPS and all that. But jeez SE, this summoner gameplay has nothing to make it really feel like a summoner. You barely focus on the pet at all.
Here's something to think about: what if you take off SMN? What would change? Well, here:
+You can now use Blood for Blood, Internal Release, Mantra, and other useful cross-class skills.
+You get an entirely new dot: Aero.
-Your pet damage goes down slightly
-You lose two cooldowns: a 5 minute nuke and a temporary pet damage increase
-You lose a very important NUKE that ACN should have because it has nothing to do with summoning (Fester)
That's it. This is the entirety of our job, right here. This is what you get for toiling to 50 and working on SMN quests. It's almost arguable that it would be better to not use SMN. The amount of damage gained through dot snapshotting Blood for Blood + Internal Release with the existing Raging Strike is pretty high, especially combined with Shining Emerald and Bane. Your mana is also now more efficient, because you are using Energy Drain instead of Fester, restoring mana to you over the course of the fight. All in all, you become more versatile, gaining access to many other class skills, including a NEW DoT, Aero, which is actually a DPS increase.
So why do I think SMN is disappointing? Because it isn't interesting. It doesn't change up our gameplay at all; we just swap Energy Drain for Fester and use two more cooldown buttons. It isn't even that big of a damage increase, coming to the point that it's almost arguable that ACN is a better choice. SMN isn't SMN, it's just a slightly beefed up ACN in a couple different ways, while losing access to many other class skills that are actually extremely useful. All in all, if it weren't for Fester, ACN would definitely be a better choice. Fester alone carries the skillset of SMN. I think that Square Enix really dropped the ball here, and they need to do something about this.