I found the summons, although not over powered, fairly useful in the game, especially against bosses that use protect/shell/reflect... mostly found late in the game. I only powered Zidane's theivery up to about 4200 dam and never used the other "cheap" characters and I found the summon damage on par with spells like flare, sword techs such as climhazzard, flare sword or any physical attack. All in all they cost the same MP and do the same damage, as they should.

And since it's mentioned earlier, I never used any summons in FFVIII because they were all weaker than my physical attacks... guess it depends on how you play the games whether they are useful or not.