Well, with the new levelcap incoming, there's room for them to make a trait for Marauder/Warrior for something like "even if it's not the killing blow, mercy stroke will heal you for X% of your hp", whether that new percentage is 5 or 10 i have no clue what would be reasonable, since it's only useable during a mob's last 20% hp and because of the cooldown on it (not that large, but large enough that you usually don't get in more than one on regular mobs, and rarely more than two or three on boss-types, at least in my experience).