A Disc Priest works similarly to a Brewmaster. A Brewmaster in 463 ilvl heroic blues has roughly 13% avoidance, 18% crit and 2-3k haste only. In this gear they are absolutely the worst tank ever. They're terrible. However, in 517 ilvl heroic epics a Brewmaster has roughly 32% avoidance, 36% crit and 5k haste. Now they become the best tanks. Similarly to Disc Priests, in bad gear your Rapture returns negative mana and in bad gear you can't blanket the raid. Even with a 30% DA the PoH from our Priests (going by Log Browser on Garalon kill last night) would be 13-15k. That's around 75k "healing" purely from the DA absorbs. In bad gear this would never happen.