I think everyone would gravitate towards the "best" build, so everyone would still be wearing and using the same stuff and materia would still be just as expensive.

The closest you're going to get for customization is the class system and even that is limited.

Don't get me wrong, I would love more customization. I loved the idea of specialized elemental mages from launch, it just never seems to pan out. Individuality is thrown to the wayside because people don't want options.