In clutch moments SS is not effective way to heal the tank. Meaning it's MP vs Reward is very low. SS is fantastic for low dmg instances as mentioned so you can focus on other things. Of course SS before when MP use has no affect because you'll just regen.

SS becomes more important later on in raiding. Coil for instance. T4 fight is great because there are different phases where the MT is going to take BIG hits while you're moving and trying to heal adds. So having stone skin on the tank during these phases can make sure you have time to get them topped off.

I tend to use Swift Cast with Stone Skin depending on fight because its a very fast way to give them the buff and switch back to a different tank and/or dps to top them off.