Mercy stroke is quite usefull. Why not take the extra amount of damage when a mob is under 20%? In fights like Twintania you will use mercy stroke a large number of times, with adds, conflags and last but not least twintania herself. You are actually gimping your DPS by not choosing that skill. Your damage won't go over the top, but it is a off GCD skill which boosts your actual dps, so why not? Don't use it as a way to heal yourself, use it as a finisher on things for extra dps.
I don't see any reason to not keep it.