It was removed part by what you mentioned but also because it was a permanent debuff. It was taken away then re-added to shadow fang as a debuff that had diminishing returns. Otherwise they could have just as easily nerfed healing debuffs all around or just for the skills in question.

Reducing the healing of a power house that is made to heal through heaps of damage by 20% vs increasing the damage by 10% for the same combatants fighting against the same powerhouse,hmm. We're just going to have to agree to disagree.