Piety badly needs a rework. There is absolutely no reason it can't get the Tenacity treatment (+0.1% damage per 33 stat) at the very least. The biggest issue with Piety is that when you don't need every last bit of mana you have in a fight, up to all the Piety you have is gone to waste and did nothing. Sort of like you got a -1000 DH debuff because your group didn't play terribly. Because of this, by melding Piety you're taking a big hit in average output, for the occasional "just incase" situation. We'll still do it when we have to, but when you realize how it works it doesn't feel like a satisfying choice.

Tenacity isn't in the worst spot but it's not in a great one either. Compared to Det the damage difference is small, so despite popular belief, having Tenacity on your gear won't affect your damage that much (barely 1%). However, the mitigation won't affect you much either. Mathematically it almost never amounts to enough to save healer GCD's. It's a fairly bland, mediocre stat. They could probably leave the damage component as it is and safely bump the damage reduction up, so choosing between max dps and extra-tanky was a viable decision.