The best way to take care of your espers in combat is to cast Bubble, Protect, and Shell upon them and use your summoner as its healer. Some people recommend fighting in tandem, but usually when you bring out a summon, it is because you are fighting something big, not a few Steelings, so being a healer is not so bad when your esper is dealing out damage and hopefully working towards their ultimate attack.

I use espers sparingly because, yes, they are still pretty much useless.