In dungeons i really dont have no time for putting stoneskin even on a tank, everyone is always in a hurry, but then again, i find it really boring reapplying it so i dont even bother.
and before some serious fights its not bad using is, but the effect is very minimal, its 10% reduction and only in the beginning.

Using it in the middle of a fight is rather worthless. Before it used to be useful since WHM had 18% stoneskin, using it on tanks was considerated reasonably good, now i feel its quite pointless.