Yeah, they can be useful in certain situations, but not that useful. xP In fact, as far as I'm concerned, most of the situations in which they're considered 'useful' by most people, I consider them to just be slightly less useless, since it's not like any of the regular monster fights are particularly hard. :P

However, it's kind of nice to summon an Esper right before using a save point, since that way you can do it without losing all of your MP, making them purely a bonus.