I agree about them being useless because for a great majority of the game they can easily be killed even by regular enemies. Belias is partial fun when you first get him, but afterwards there was no point in them, besides having some extra challenging battles and extra MP. They weren't even story related either which made them less memorable. You said something about having to fight them is a minus? In several FFs you had to fight at least one summon before you get them like Anima in X, almost all of them in VIII, I don't think any in VII, but I have to play that again, and I definetly know in IV you had to fight several in that as well.