Roar!! (sorry 7 coolers~that means i've been drinking...)
umm.. yah.. i hardly ever use my summons either, but I found that one of the Biggest Challanges in the Game is to try and get Evey Character to Learn the Zodiac Summon... Ive done Some Reasearch And i believe this to be the most time consuming and difficult of any of the perfect game challanges.

There are more difficuilt challenges out there like the Ramza (Calculator) only no Spells learned Challenge.... but they are for perfect gqme challenge, they are just player skill challenges.

P.S. I have attempted to get every party member the Zodiac Summon only once and I have Failed.