Piety provides slightly less than 8 mp per point; we'll call it 8 for ease of calculations. You get 2% of your mp back every 3 seconds in combat, so 40% per minute; adding Aetherflow to this makes 60%. This means that as long as you don't lose any mp from regen or Aetherflow because your mana is too high (and since this topic is about SMN mp issues, I'm assuming your mp won't be that high), every point of Piety you have will grant you 8 mp plus 4.8 per minute of fight length. In a ten minute fight, you get 56 mp from a point of Piety; a fifteen minute fight means you get 80 per point. If you throw 30 Piety on your relic, that's 2400 mp over the course of a fifteen minute fight. Whether that's worth sacrificing 30 crit or determination or whatever you would otherwise be putting on your relic I cannot say for certain.
I doubt it though.