I'm more surprised that there are whms out there not taking advantage of stoneskin. I tend to use it a bit to help mitigate damage, both on tanks and dps. It doesn't offer as much help on PLD, but I can usually pop one pretty easy in the middle of a fight with Swiftcast -> Stoneskin to avoid some burst damage and pair it with Regen instead of blasting cures to get them back to full.
They sit at half to 2/3 health and come back up while taking next to no dmg. Even with the mp cost, I haven't really had any issues with running out, even during some longer battles or bosses.
Helps on PLD as well when you take into account mitigation. Might only defend some 500+HP, but under mitigation it goes a longer way. Anyway, stoneskin = great for pretty much anyone. Who can argue against no damage?