A pld without stoneskin is pretty much a /kick after a certain point and will only get more important. With tank swaps being a "every day 2.1" thing. The tanks will almost always have a point where they need to provide some damage mitigation, and stoneskin is the "go-to" spell for that.

Let's take ultima (realistically you should be doing it as early as titan HM and coil1,2 etc) For half the battle the OT will most likely be on sword oath, and they have a duty of casting stoneskin on the MT, do their riot for mp, and repeat. While people have long used gear to brute force it, that tactic generally smooths the "oh shit" possiblity.

Ultima HM without tanks providing stoneskin = tank purge and/or crits, at wrong timing(stack change) dead tank. With stone skin, no problems, as the tank will easily be topped out without the extra strain on the healers (laser's and ifrit being the worse time for tank purges).