Sure, I have 3 main reasons. Firstly, I have tons and tons of "battle" materia. Yes, I can sell for chump change, but If I as a player dont need them or buy them and just get via normal play VS. gathering & Crafting(the worst). Second, when was the last time materia was updated? I'd rather burn through my tier IX and X to get a "rare" XI, then sell them for 100-500gill each. And think about it, Materia price would rise in those tier and become more in demand.