I think it actually moves your hitbox during the animation, hence why you can't "jump over" aoe puddles without getting the dmg/dot/debuff using your jump skills.

The enmity drop seems quite useless imo, where would you even use it? During the start of the fight you can't really use it until you build up aggro, unlike quelling strike used by casters, and by that time you build aggro, if you steal it from the tank then the tank is doing something terribly wrong. Mid fight you shouldn't be able to catch up with the tank's aggro lead from their opener and occasional use of enmity combo even in dps stance, so it's kinda useless randomly using it.

If by any chance both of the tanks die and you're holding aggro, then yes it might be somewhat helpful when one tank tries to get the boss aggro back after they get raised, but any decent tank should be able to easily grab the boss from a dps using tank stance and enmity combo considering how much enmity multiplier they have. Also if you use elusive before they provoke the boss, it will just switch to the next dps/healer, so you need to make sure the tank has provoked.