When Off-tanking I view TBN as a tool of convenience, assuming there is no significant loss in potency. It's an instant on-demand 50 blood when used correctly. This means you can set up Blood weapon + delirium immediately on-demand. Using it this way means more blood weapon time since you spend less time delaying blood weapon to ensure you get enough blood for Delirium before blood weapon falls off. Unless you have some perfect convoluted method of always ensuring you have 50 blood when you activate Blood Weapon, I would say this compensates for any insignificant loss in potency by using TBN.