Piety is useful for ASTs outside of high-end content simply because their MP economy is pretty bad, and they have no other methods of managing their mana outside of Lucid Dreaming (for Refresh) and Lightspeed—which, every other use has to be used in tandem with Sleeve Draw if one was trying to utilize the latter in an optimal fashion. So they have one-and-a-half ways to manage MP, where as WHM as three (Lucid, Thin Air, Assize), and SCH has three (Lucid, Aetherflow, Energy Drain).