I like the idea of having this restricted by actions rather than hate.
i.e. If you attack, cast, are attacked, or are cast on (by a mob) during BW, then the BW fails.
This prevents you from BW while fighting, but allows you to Root/Mez/Snare and run away from a mob and stop the bleeding from that nasty gash on your shoulder.
I think this would remove the overpowered ability to have 4 casters BW the tank while he is meleeing.