PLD should never be casting clemency unless the healer is dead and you're preventing a wipe on a pull that can still succeed. You flat out do more damage than the healer (assuming no gear/skill discrepancies of course), which is what you should be focused on, and clemency eats up a bunch of MP that would otherwise go to your casting rotation. It sucks, I freaking love being able to be a backup healer, but that's not how this game is designed unfortunately.
Now if you're a GNB, that's a different story, but that's probably moreso that they don't always need aurora for themselves, and heart of corundum is an almost spammable get out of jail free card for a party member, and both abilities have the benefit of being OGCDs.