I checked last night, Wandering Plague hits for 75-100ish on beasts, with 25% less threat it comes down to let's say 50-75 threat. Even a threat-reduction-specced-healer will do more threat.
So that's just like Heart Strike, the threat is so incredibly LOW, nobody should care about it.
If the beasts stay near the melee group, OR the tanks fucked up by using a high threat AOE ability, OR the ranged are just playing shit.