There's a debate going on about how good/bad Summoner is for Endgame. I'm firmly planted on the good side. SMN is highly versitile and can do things no other class can do by taking all the chaos and guess work out of parties. Just follow me through Aurum Vale and I'll show you why you would want SMNs in your endgame parties.
At the beginning, it's not that impressive. In the dungeon you have a lot of trash that you should be sleeping/assisting the tank. This is where SMN is the least useful. The Summoner should be using his Ifrit DPS Egi and should not be using Bane because it wakes up the sleeping mobs and makes hate management a pain for the tank while at the same time slowing down your DPS meter. Do not want. Use Fester if you have a chance. If you really want to use Bane, use it when there's only one mob left to go and the other mob is 90 percent dead. That's an easy way to pass on your DoT without recasting the spells which is a waste of time and lowers your DPS to agonizingly slow levels.
The only time you want to use Bane is actually when your DoTs are most useful and that is with the Diremites who cannot be slept and hit hard. Piling DoTs onto one diremite and passing that DPS to the rest of the group really helps your healer and DPS because it gets the mobs down quickly by whittling their HP over a good stretch of time.
In the Locksmith Room, put your Ifrit away. It's too squishy for this. Use Titan and use it wisely. Don't be a pain for the tank by letting your Titan pull hate off. Use Steady until your tank is good and hated and then let your Titan go fight. Use Sustain, but Titan's shield is enough to stave of 8 stacks of DoTs. Also, cure your own pet. Please don't make things more difficult for the healer.
Choose Ifrit for the dungeon trash again.
Next is Coincounter. Again, put your Ifrit away. Coincounter kills Egis at close ranges. Use Emerald Carbuncle. Coincounter will target party members with a strong "Glower" attack so you want to "Place" your carbuncle away from PT members and yourself. Your Carbuncle will then nuke non-stop, making very short work of Coincounter.
Choose Ifrit for dungeon trash....
Finally, Miser's Mistress, where your Titan Egi really has some fun! Use Steady at the outset and keep your Titan next to you so it doesn't accidently get nailed by the AoEs and die. When the Miser starts getting low, she uses Sow. "Place" your Titan among the Seeds and "Sic" the Titan on them. Voila! Instant off tank!
This makes getting rid of the sprouts so easy. Your Titan is freeing up attention for the Tank to focus on Miser, for the DPS to focus on the adds and for the WHM keeping everyone alive. And it frees you to use your AoE attacks.
Miser's Mistress goes from being a stressful pain in the rear to super fun in a matter of seconds. Go Egis Go!