Depends on what you mean by viable, and I haven't actually seen any top dk tank that isn't using BT so feel free to link them
. The only advantage there is to not using BT is that you don't need to micro it, and you lose a very significant chunk of survivability (both control, and a good amount of death strikes). The claim that RC is more dps that I see a lot is simply incorrect since you always have the option of turning some of your death runes to heart or nectrotic strikes, or simply use some more dps gear due to the survivability added by RT.
The difference between BT and RC isn't even close to two stats simming at .89 and .90, it's very significant (not that I'd see a reason to gear for the .89 stat over the .90 stat either, unless it's better in specific situations, e.g. movement or aoe). RC is worse than BT in every situation, in some just slightly, in most significantly. That's black and white to me.