Clemency scales off of STR. At ilvl 150 I currently heal for ~3000. I have not had my cast interrupted yet--it's always seemed to me that few things will actually interrupt a PLD cast from my experience with Stoneskin, which I've used a lot as PLD. Consecutively I can get off 2 Clemencys and have some MP left, but between Riot Blade now getting used more frequently and the MP bonus from Shelltron, we regenerate MP more regularly and a little faster than we use to (PLD note: unless you have a planned purpose for Shelltron, be using it on CD).

I definitely prefer the PLD heal to the WAR one personally, for the same reason I prefer Shield Bash over Brutal Swing BUT Clemency is really most beneficial in those heated moments of "omg that person needs a heal or they'll die!" and with a 3s cast it's probably going to fail a lot, so between the two its really a personal preference. And on another comment that was made in the OP; it wouldn't fit the job MO for WAR to heal others in the party, whereas its kind of expected of a PLD, being that we cross with CNJ.