By all means, you should get them just to listen the the track 'Esper Battle'. The fights against the Espers are often fun and challenging themselves. It's just that the Espers really aren't too useful. The only boss battle I ever found a use out of them for was when (SPOILER)I fought Vossler in the Leviathan. Belias quickly made work out of him.

Quickenings are much more useful, but even they lose usefulness later on when you'll want to conserve your MP for spells and other abilities.