It sounds like there are a few ways they could make clemency more useful. Granted, I have not actually played a Paladin at 60 yet, however I have extensive experience with tanking at high level in other mmos.

A few ideas from other games that could work well here.

1. It definitely sounds like the cast time needs to be lowered to 2.0 seconds, and made non interruptable, or at least harder to interrupt
2. A group heal with half the potency is another option
3. A heal over time could also be interesting
4. Finally, in one MMO I played called Vanguard, one of the healer classes had a "reactive" heal, that they could cast on someone, and it started with 3 stacks. Each time the person takes dmg, the reactive heal triggers, but at a fairly low potency. So perhaps in FFXIV, it could heal for 1500ish each time someone gets hit, up to 3 times. Interesting, helpful, but by no means game breaking, since you'd still only be able to use it twice with one mana bar.