Because it's no longer unqiue. Scholar's have emergency tactics, which can make their adloquium heal for more on a crit than even our own criticals (it turns the barrier into a heal, effectively a 900 potency cure). Astrologian has Essential Dignity which can deal a 1000 potency cure to anyone low on HP, and can use regens as we do.
The argument against stoneskin is that it's somehow broken at high HP levels. What Yoshi fails to consider in his argument is that damage is also significantly higher at level 60, so the total mitigation remains about the same. All we get in exchange for this potency is the weakest of speed boosts to casting regular stoneskin (not even stoneskin II), and a stoneskin ability which is no longer worth its casting cost.
