1. It only has applicability in PvE. Every other class feature has some applicability for both.
2. Aggro is binary so Sooth is binary. i.e., it either works or it doesn't (mob either aggros on you due to your heals or it doesn't). If it doesn't, then you get absolutely no benefit.
Unlike the other class features which provide some benefit in ALL situations, Sooth might provide a benefit in SOME situations.
I agree with you there. The problem with aggro as it stands is clerics are the only ones that are producing threat on every mob in an encounter from start to finish. 20% less of something will always be greater than the 0 everyone else has generated when adds pop.
Comments
Sooth reduces threat, which is not binary.
If Sooth doesn't reduce the threat below the threshold that triggers aggro, then it really doesn't matter how much Sooth reduces threat.
Just like most of our healing, they need to move everything to being active.