You might not like it but at least Tenacity has it's uses. A full Tenacity build takes roughly 8-10% less damage while only losing out on 2-3% dps vs a crit/DH build.

Tenacity builds are GREAT for new expansion leveling dungeons when you have no idea who you're going to be grouped with.

If I ran the design team:

Tenacity would be a tank stat that would affect healing received (instead of the current use which for some stupid reason increases healing done by the tank to themselves)

Piety would affect MP regen rates and healing done.

Det would go away.

Tanks would get: Tenacity, Skill Speed, and Crit
DPS would get: Crit, DH, and skillspeed / spell speed
Healers would get: Piety, Spellspeed, and Crit

Point is, if Piety and Tenacity are going to be in this game they need to be worth a damn and right now they're not except in extremely niche situations.