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

[Shizuna]

Well, thanks for the reduced MP cost SE...

Yeah, I agree with this. There could have been a few more ways to buffing SMN without affecting SCH. I mean, there's all those skills (ie: fester, tri-disaster, enkindle) and egi (ie: ifrit's radiant shield) that needed to be changed/fixed and all we get is some basic pos buff that will increase our damage by like 5 dps if we can even book smack more. In some instances, I, for one, can't do that constantly in t10, 12, and 13, and even if I could ruin II+book smack, there will probably still be mp issues, so either way this doesn't even matter much. I should just change to BLM right now cause I have the i135 dreadwyrm weapon so why should my SMN matter anymore... :/

..so it's time to add book smacking to my rotation. Smh :/

You haven't for all this time?? Or are you joking? ._.