Stoneskin is very, very nice to have, but ultimately not needed for a good portion of the content. However, i would say that its interests skyrocket for titan HM. In this fight, you will take regularly (every 30 seconds more or less) a very predictable spike of up to 6k damage (to your about 5300 hp pool) in the space of two seconds. While precasted cures landing in between, parry, block, dodge, sch shield and CD can all help survive this, you have a nice window of about 5 second to cast stoneskin before every burst of damage, making the whole thing much less stressful for yourself and your healers.

Mercy stroke is a joke and has fringe interest at best. The odds of you landing the killing blow on an adds during a boss fight are abyssmal if you were to try and time it perfectly, and you will have something else to think about than to try that. During trash, you may have more luck with it... But it's trash, so who cares ?