Clemency is actually quite powerful. For example, during the 4x Jagd Doll phase of A2S you can pretty much replace your healers for a good 15-20S with Clemency by casting it on your OT in between cleaves if you have good timing.
It's just easily interrupted (especially by magic it seems) so you really can't count on it and it ends up being mostly unused (cancelled casts interrupting combos makes it especially hard to justify risking the cast). If it were reliable it's a fantastic skill. I believe the most I've ever healed for with a single cast was 14k on the target and 7k on myself for a total of 21k heals. It's especially nice that you get to double-dip on Convalescence/Defiance/Mantra so that each of those becomes a 30% buff to Clemency's potency.