I really didn't like the times that 1 character had one (or up to two in the case of FFXII) assigned to them, I thought it worked a lot better in say IV or IX when you have dedicated summoner classes who may have secondary abilities to cover the difference when you're not using them.

Usually in games like VII I'll have on character boosted up with Summon Materia, or give just a few only to characters who already have extremely high magic.