I think TBN is great.

You can use TBN and Dark Mind to deal with most tank busters in this game. Unlike Inner Beast, TBN doesn't need you to be in tank stance. Since tanks in this game are mostly in DPS stance, TBN is realistically a leg up over WAR in terms of personal mitigation. As far as comparing it to sheltron goes, TBN won't suffer from diminishing returns, so while it doesn't provide as much raw mitigation as sheltron alone, it has much greater synergy with other cooldowns than sheltron could ever have.

Combine it with the fact it's dps neutral compared to Dark Arts, AND on a 15 second cooldown, TBN is a top-tier cooldown. No way can you call that bad design!