i think viridiana you don't know well how work the enmity into the game...
if you don't attack a target, as ninja, your enmity will be 0. since we don't have heal or taunting capacity that generate Enmity in area without dealing damage. the transfert of enmity will tranfert our enmity for each target to the tank, if we attack only one target, while you have multiple target, only for this target the tank will get something.

meaning for a monster that the tank can't attack (generally because invinicible) we can't give him enmity since.... well we don't generate any enmity to him.

the enmity is:
- Damage = % of enmity.
- heal done = % of enmity to all the target in fight.
- Taunt = get first on the hate list of the monster.
- Flash = % of enmity generate for each target affected in the area of the skill, without deal damage.

soo far this skill, like i have said have little to no real function outside give us something... because they had to give us skill. if the tank can't attack a target (because too far), it will be pointless since the tank enmity will not move from what we did give him. the tank will need to attack for get more enmity. if the monster is invincible, we will generate 0 enmity, then nothing to give to the tank.
if that was a transfert of hate positon, why not.... but that not the case, it's a skill more situational than what you think.