Can someone give me an in-depth explanation why Summoner is arguably the worst DPS?

[Akiza]

I'm no healer, so I'm not sure how much truth there is to this statement. (Being a good Healer looks ridiculously hard IMO) But in saying that, as long as you know you class very well as SMN, all the fights can be broken down to match up with SMN CDs. Even if the fight is brand new there tend to be a lot of obvious areas to use skills. The only one I had trouble with in FCOB was T12, because I was trying to match my dots up to Bane onto Black Fires and Bennus both. After swapping to the SCH doing White Fires and me on Bennus it really simplified the rotation. (Now I don't even Bane onto Bennus, but rather M2, Bio, Virus. Ruin Ruin)

SMN has a lot of patterns to it, which IMO does make it easy to master on fights, and makes it extremely adaptable.

That's my point other DPS don't have to plan out everything they react to what happens. Even White Mages don't plan out every phase it's just easy to tell when to pop big heals as oppose to when to use CD's on Summoner. Healers have a better flow then Summoner so even though White Mage is harder to play it's easier to know when to hit medica 2 or other big heals while most Summoners screw up the timing and get punished harshly for doing so. The Summoner is just broken compared to other DPS primarily because it's more difficult to use effectively then a White Mage in endgame.
I guarantee if asked in shout which job is more difficult to play between White Mage and Summoner most players will Summoner is more difficult to play because it's harder to keep up DPS for long periods of time which makes Summoner the Hardest DPS to play.