Flecky why do you sound like you want to find a fault in his post?
About the espers, I agree they are mostly useless later on. But in chapter 3 you can make very good use out of Anthanex against the epic boss. You just need to use Galeoths spell book spell, the first one that absorbs incoming damage. I think it's called glacial shield or something. Summon Anthanex then immediately use that spell on him. He can last a very long while.
I can usually use that spell twice on Anthanex and keep him alive for 80% of the epic boss fight if I move correctly. He can almost kill the epic boss himself.
I never really used them after that though. Tried Faerga against the healer boss and he got totally destroyed in seconds, completely useless!
Aero needs to buff their stats probably.