I actually use stoneskin a lot. It's usefull when 2 or 3 party members are taking damage, I will throw a stoneskin at the tank then heal the others. It is a great damage mitigator - if only briefly. I find it more effective than media if only a few need healing. As far as mp costs? use shroud of saints as much as possible. If not, you can find yourself low on mp even if you don't use stoneskin.