Since cast between fights it's free mitigation that lasts until used I can't say it's useless, but it's certainly not very strong. Even on the ideal target, a 20k health warrior it's still less than a basic heal.

They should give whms back their 18%. The scaling health fears really don't hold any water. As health scales you're going to make enemies hit harder. And you're going to make healers heal harder for that matter. Keep things balanced and it shouldn't be a problem. DON'T keep things balanced and you've got bigger problems then stoneskin potency.