Clemency costs more than 1/3, so unless you have PIE on your gear or spec into PIE with your bonus points, you can only cast it twice before you have to replenish your MP. It takes 10 GCDs (~25 seconds) to recover enough MP for one cast. With Sheltron you shave off 5 seconds.

At level 60, I have 5195 total MP, Clemency costs 2121 MP. Riot Blade (combo) recovers 442 MP every 2 GCDs. Sheltron recovers 707 MP every 30 seconds. At iLv 166 I heal for 3100 HP. Seeing that at iLv 178 it heals for 4100, it seems to scale quite well. But still no way replacing a healer.

Interrupts are pure luck. I've gotten interrupted twice in a row before by bosses. Trash rarely interrupts Clemency casts because they simply don't hit hard enough. But boss skills has a real possibility interrupting Clemency casts.

I find DF healers rarely take advantage of Divine Veil, though...quite sad.