The purple ones are important, since they affect cool downs. But the yellow ones... I mean, if you're needing Piety as a healer, maybe just get it from gear. I'm all for scrapping it entirely. My main issue is that lower tier materia are practically worthless outside of overmelds. At least give us the option to upgrade them to a higher tier. I wouldn't even mind if it was something stupid like a 10:1 ratio.