piety is pretty useless.. part of the sch job is keeping on top of your aetherflow and MP.. you should never be out of MP.. the only instance i tend to get low on MP in is Titan HM with his stomps because you have to spam Succor, for this reason priority for my aetherflow stacks goes to Lustrate (for MB) > Energy Drain .. during the Gaol stage where there is no aoe and the tank is topped off i will burn all aether on energy drain and then restack.
other than that like i said mp should never be a problem for sch