Cower is as close to useless as useless gets. If it was off the GCD AND didn't cost Energy, it'd be worth weaving into some rotations. If it dropped all aggro but still used a GCD and cost Energy, it'd be worth using. But a 10% threat drop that costs Energy and a GCD is useless.
Pulling threat through a Hand of Salvation is pretty embarrassing. Doing it twice on consecutive pulls on H Shannox is enough to make me want to just auto-attack for the whole fight. :X