I feel like you can't take these things in isolation. Inner Beast is 450 potency with Defiance and SE buffs (compared to Grit Bloodspiller's 475 potency), is a 360-450 potency self heal, and a 20% DR move that doesn't lose effectiveness if it soaks up autos before the big hit. What makes it "useless" is the fact that Fell Cleave is a 600 potency attack with Deliverance and SE buffs. It's the same situation with Unchained; on any other tank it would be incredibly powerful. It just so happens that WAR has an even more powerful ability on top of that which is mutually exclusive. #firstworldproblems

That's not to say that TBN is perfect either. Use it too early, and it gets eaten up by autos prior to the hit you actually want to mitigate, and mistime it and you get a 168 potency loss (185 potency in Grit). It's easy to say "just hold on to the extra MP", but when your goal in a buff window is to fit in as many DA uses as possible, hanging on to 25% of your MP bar just in case you need it is going to be a dps loss.

The addition of TBN also devalues DM, because the only thing less appealing than hanging on to 25% of your MP bar to use TBN is reserving 25% for DADM which is a flat 168 potency loss. The DA effect might as well not exist now, and a 15% magic DR cooldown is not an acceptable substitute for having both ToB and RI on WAR. That's before we even attempt to make a comparison between Vengeance and Shadow Wall, or Holmgang and Living Dead. There's simply no comparison. WAR mitigates more for less effort, less resource management, and loses less dps to do so. It's just more forgiving at all skill levels, and there's four weeks of raid performance data to support this.

Removing all the resource costs from WAR's stance dancing on top of removing the costs from IR/Unchained was what pushed this over the edge. They had a slight advantage over DRK prior to going into 4.05, and now it's a significant one.