If we could specifically use something like materia to increase MP, that could potentially be an interesting form of customization. Like for BLM, having a bit more MP might mean that when things get wild, you'd be able to cast Fire an additional time without sacrificing your ability to hit Flare Star. Deciding whether that was worth the materia to have that flexibility might be an interesting choice.

But that said, I don't see anything like that happening, unless they get really wild in 8.0.