Inventory is already pretty pressed for many players (myself included). I don't want to speak for all players but i know myself and those i play with lug around all sorts of sets of gear and are lucky to have 20 slots free (including rets) at any moment, Materia could easily chew this away and then some. From what i have read it sounds like there will be all sorts of combinations of Materia (both affect and rank ranges), and with all these various Materia.... inventory could fill up really fast.

So please consider adding a tab to the inventory UI that stores just Materia.