Piety does do something contrary to popular belief. Aetherflow for SCH and Assize for WHM both restore 10% of your total MP so the larger your MP pool the more return you get from these spells. Your Piety also affects the rate at which your MP replenishes itself during battle. Right now it seems impossible to run out of MP with how how much Piety is forced on you across all healer gear so running completely dry at any item tier is a sign that your are playing inefficiently or everyone else in the party is eating every AOE ever and enjoying the Resurrection animation a bit too much.