Except it is not. Just compared two God Kefka parses where I did about the same DPS with both PLD and DRK (± 100 DPS). Holy spirit was 11% of the total damage dealt. Bloodspiller was 18% of the total damage dealt (and Souleater was like 20% total damage). For both PLD and DRK, auto-attacks are what deals the most damage.
Fell Cleave cannot even be compared to Holy Spirit or Bloodspiller, since it's about 40% of a WAR's total damage (more than twice Bloodpsilla's contribution, and three/four times Holy Spirit's).
IMO, TBN is a bit like Sheltron but with a higher risk. If you use Sheltron but you aren't taking any damage in the next 10s, you're losing 100 potency from Shield Swipe (happens often when being OT if there's not that much AoE damage for example). But if you're using TBN and you're not taking enough damage, you're losing 50 blood. Now that TBN is 7s, it's kinda hard to not make it break, it's long enough to cover 2 boss' auto-attacks which are 10-15k damage each usually. But if it is timed poorly, just before the boss casts something, it may not break and that feel bad.
My point is, TBN is a great ability if used wisely, otherwise it's kinda disappointing. A ressourceless, 13k HP shield every 15s would be rather OP, so it has to be gated by a ressource. But DRK only has 2 ressources and both are used to deal damage, so you consume mana (you lose damage for mitigation) and if you shield yourself at the right moment, you're rewarded with blood (which makes TBN virtually free).
If you time TBN poorly, you're not only losing damage, you're also using mitigation for nothing (shield isn't breaking = you didn't even mitigate 13k damage).
If you time TBN well, you're not losing any damage, and you're mitigating at least 13k damage.