It's just useful to have when fighting trash enemies or during XP/spiritbond groups. In non-boss situations you can usually throw up stoneskin on everyone, Regen whoever takes dmg, then go to town nuking. It's better than just standing there at full MP. On older bosses like Deepvoid Slave and Ifrit you can get away with wearing nuking gear and speed up kills significantly/kill nails/cover dmg because your pickup members can't stay alive/etc.