Unfortunately, you have to balance the game around bads when you consider the tank role, because a tank that decides he wants to burn B4B as MT is going to wreck a run by instantly disintegrating and taking the healers with him. Yes, good/smart tanks will use B4B appropriately, but think about how many awful DRKs there were, and now imagine they had B4B on top of that.
MORE DPS! WAIT WHY AM I DEAD? - every tank forum thread after making B4B available to DRK and PLD