Clemency is basically only useful if the healer dies. In those situations, it's incredibly powerful, but otherwise it's a useless skill. Since healer DPS has been nerfed into the ground you're always better off using your MP to DPS.

It's unfortunate in a sense because PLD effectively is the only job that won't self-heal at all playing optimally, but on the other hand it's also the only job that can spam self-heals.