They are pointless.

So many times I've summoned one, then I head away from the danger to try to cure myself somewhat, and the stupid AI of the esper means he follows me...I'm thinking "Nooo! Attack the Boss! Dont bring it near me >_<"

Anyway, further on into the game they literally take one hit to die, they are a waste of MP in my opinion, just like Quickenings are (unless you know for a fact your quickening is going to finish the Boss off).

You collect them for fun only I guess