I don't think having one summon per person is that bad... in fact, I'd like to specialize characters with skills only THEY can use. It makes them more irreplaceable, even the ones who are terrible stats. I guess it's like a drive for me to make all of them equally uber-strong.
Since the game allows customization, then the specialization would be up to me, which is somewhat more fun. That's just me, anyway.
As far as I know, for summons, there are Hashmal, Madeus, and Ultima. Who are the other 3?