I'd much rather see a reform to how materia is stacked then to put them into the seal exchange program. As it stands now every discipline has their own unique category for exchanging seals Crafters have items, Gatherers have materials and Fighters have NMs/dungeon loot. Adding materia to the mix would mean that they are admiting that x materia is useless. The reason why items/materials are exchangeable is to the control the flow of materials that enter the market, but even then it takes 0 seconds to cap mistletoes upon resetting at the Grand Company at a 1700 item cap.

It be much better to consider ideas that would expand materia as a whole then one that's just used to clear unwanted materia.