I used to have over 2000 mana at one time and it gave me the luxury of not having to use it efficiently. But as of late, MP has been limited to 10k. And I find with good gear, I have more than enough Piety to support 10K MP without the need of adding any Piety Materia. Unless someone can point to some advantage like MP regen or something that has changed that i'm unaware of, I would say there's no real use for Piety materia unless your gear isn't that good enough to support the 10K cap.