It's mainly used for progression to squeeze out every last bit of mitigation for tank-busters. Or if you want to save a weakened DPS from a stomp/cleave. If you're not doing anything else and not MP-starved, why not? You should Stoneskin II + protect at the start of every instance on WHM because it's free... so again why not? If you're running AST/SCH or something don't bother - I normally just Protect and stoneskin the MT then call it a day.