I'm not certain about whether it's OP. For me Clemency is a dual purpose skill, in solo content it's point is obvious - self heal. In group content, especially for OT, I think it's more valuable as an "Oh Crap" heal for the MT (or self). Alernately, the OT can keep half an eye out for trouble and do something positive if things are fraught and health is low - say if a healer is down.

Combined with Divine veil, Cover and Intervention, I think it's a part of the Paladin's party defense toolbox rather than straight self heal. I mean obviously you get 50% of the heal yourself when you use Clemency on others, but given the MP cost, GCD loss and combo interruption, it's not like you're there to be a healer, it's just part of the toolset. If you spam it too often you hurt your DPS, a lesson I learned when a friend commented during HW content at how often I use clemency.

The fact that it takes away from the DPS counterbalances the power and spamability of Clemency IMHO.