Tenacity's mitigation is a non-factor. That it offers nearly comparable output to DH is nice, but DH still edges out in the end. DH translates to stronger damage and stronger healing (both from lifesteal and clemency/equilibrium). If tenacity offered better passive mitigation in exchange for worse damage than it gives now, I'd definitely use it. But if it doesn't save me a cooldown, and saves healers maybe 2 cooldowns an entire fight, it's not worth the DPS loss.