Most people's problem is that Espers aren't quite the PUSH BUTAN WIN GAEM Aeons of X. They aren't for boss battles or hunts, they're for either power-levelling (only the summoner will gain EXP given by the monsters killed while the Esper is around) or dealing with mobs.

And for this purpose, they're fine all through the game. The once I got badly mobbed in Henne heading for Zodiark, Ultima was a life-saver.

Solo character runs make a great deal more use of them as well, I've noticed.