Basicly anykind scatter effect in power makes it "useless" in teams. Players don't wanna enemies to run away, because it makes they life harder. If I would start using Spectral Terror in teams a lot, I would be kicked out of team. So is power good if other players would kick You out?
As long Spectral Terror cause enemy to run away, _most_ Illusioinist will avoid using it. Sure it may look funny, but that does not make it usefull. If it's not usefull, players don't take it or don't use it so much.