They're definately useful to a point. Don't expect your level 1 espers to continue holding their own after a while, move up to level 2 and eventually 3. If you can get one around the enemies' levels, with an element they're weak against you got a great combo. There's a whole guide on using them well on GameFAQs, granted not every esper is worth using, but you can't say they're unusable.

Hunting down the non-mandatory espers and collecting them in the bestiary was alotta fun.