I've tried using it in 4-mans, tried testing it in several different situations.

With 1 stack, I find the healing negligible. It's less than 5k unless it crits. That's not even the amount of HP done by a boss's single unavoidable AoE attack, such as Yojimbo's Metta-giri or the first boss's Kenki Release. Medica2+PI will indeed heal everybody to full, but then again, just Medica2 alone would do that after a few seconds (nobody should be getting hit but the tank anyways under normal circumstances), or indeed, a Medica will fall just short of healing everybody to full and natural HP regen should take care of the rest, or just let them go until the next (that tiny amount won't make or break anything).

Getting 2 stacks is nearly impossible without over-healing which means some people will be at full HP and only 1 stack, and some will be at just shy of full HP, with a Medica2 buff on them if I do a Medica2+Medica trying to get 2 stacks. By then, everybody is already full and I have no need of PI anyways, AND what Confession people do have will wear off long before anybody but the tank gets hit again.

Even at 2 stacks, the amount healed (the few times I was successful in actually getting a PI off when someone actually needed some healing) is still paltry at ~7k. Myself, as a healer at ~300 I have 25,000 health. 7k is about 25%. Big whoop.

Metta-giri, for example, hits for 10k.

So I'm going to go ahead and say that it is going to be useless in 90% of all situations in a 4-man dungeon. I could MAYBE get some use out of it whenever you got something like Akh Morn going on... oh wait, I don't HAVE PI in Final Steps. Maybe if I ever ran into a Shinryu in my roulettes? *shrug*

Seems kinda silly to have an ability that is 99% focused on Savage Raids, which a small portion of the playerbase is even interested in doing. Do other jobs have similar abilities that are only useful in those situations?

I suppose I've yet to find a need for the Lv70 PLD ability, though it sounds awesome on paper. It's kinda like cover -- trying to actually get close to a DPS who is about to get nailed is not the easiest thing to do and will oftentimes put yourself at risk, 'cept for maybe something like the first boss of Bardam's. Again... you don't HAVE that ability at that point because it's Lv70.