Quote Originally Posted by waterboytkd View Post
I think this is the important point. The MP cost means you're giving up an Edge/Flood for TBN. No other tank is giving up its offense just to be able to use its defense*. If you use it correctly, it breaks even. Risk for no reward.

Considering DRK is also bringing up the rear in the dps department, I think this is a fine suggestion. Another would be you get two stacks of Dark Arts, not one. The idea being: MP you could otherwise spend on offense gets spent of defense, but if you spend it wisely/best, you will actually get a boost to your offense (an autocrit Edge/Flood, or 2 Edge/Floods).
If you use it correctly, you drastically reduce the damage that you take. So you need less healing. So healers can an extra couple of GCD's on DPS.
Though I do kinda agree that Dark Arts should also guarantee a Crit Direct hit.