Is TBN a dps gain in grit, yes most likely. Is it actually a gain when compared to stance dancing? Nope. Again, my reference is 8 man content, though I think this can be easily extended to dungeon bosses easily as well since the tank busters are minimal, with proper mitigation, and the damage they do otherwise is small and managable with a shield or regen.

Now compare this to paladin or warrior, who pay nothing for their buffs except following their rotation, and have mostly free mitigation (exception being warrior who seems to have to pay a very high cost for some of their kit). Then realize that after paying the penalty of not regening your mp, the cost in mp of boosting your damage, and the mp cost of mitigating using TBN. Taking all those costs into consideration, realize dark knights mitigate less than both tanks with on demand mostly free mitigation, and do less dps than those classes who have no penalty for their self buffs. This is not balance.