Mercy stroke is ok. I have it but I don't actually slot it. Stoneskin is fun because I can find interesting places to use it:

I'm OT, and MT is about to take a big hit or healers are getting behind.
Someone just died and I want to help them out after a raise.
I'm OT and about to swap in for MT.
I'm MT and the boss is wasting time not attacking me (Titan is jumping/landslides, for example).

My favorite was putting up a stoneskin on the demon wall right before it repels. I watch the rest of my team go flying back while I don't budge. Everyone kills that too fast now though.