It's not a single effect, there is a damage component. Combined with shadow strike to keep this up the entire time of a fight, it's actually your most efficient dot, by alot. Especially when the fight duration reaches ridiculous levels 90sec +. 15+ ticks, at level 34 that's about 150 damage for like 30 mana. Also, snare is useful for more then fear kite, stops runners in dungeon which can lead to wipes, also in a group snared targets are preferable, for aggro control, and emergencies. If your wizard pulls aggro it's alot easier to snap it back when target is snared.
The way it should work, is damage component remains, snare part is ignored. If it does not work like that, it sounds like a bug or oversight.