Why TBN is a powerful but poorly designed ability.

[Kitfox]

When people are saying TBN is a dps increase, are they talking about tanking in Grit?

Am I missing something when I average this out?:
Outside Grit:
No TBN → ((150+250+280) x 2 + 140) / 6 = 250 potency per GCD (Souleater x2 with one Dark Arts)
TBN on CD → ((150+250+280) x 2 + 400) / 7 = ~251 potency per GCD (Souleater x2 with TBN + Bloodspiller)

In Grit:
No TBN → ((150+250+280) x 2 + 140 x 2) / 6 = ~273 potency per GCD (~(Souleater x2 with Dark Arts x2)
TBN on CD → ((150+250+280) x 2 + 650) / 7 = ~287 potency per GCD (Souleater x2 with TBN + Dark Arts + Bloodspiller)

Obviously TBN is not meant to be used for AAs to gain DPS because of the 5 second duration and it seems like major oversight in design that it can be used like that. To be honest, I'm not sure I understand why Bloodspiller is so valuable in Grit besides that they wanted a cool way to bring Grit DPS closer to no-stance. There has to be a better way to achieve the same result without the pitfall that is TBN and Grit Bloodspiller.