Mercy Stroke is a tank ability, as is every DRK cross-class ability. In addition to DPS, it potentially brings with it a built-in self-heal when timed correctly. While I wouldn't give up Foresight to bring it along, I'm more than happy to discard Bloodbath to do so.

Surviving when playing as a tank class is the absolute bare minimum of what you're expected to do. You don't get a gold star for mitigating attacks with the correct amount of defensive cooldowns. Nobody will give you a standing ovation for understanding snap enmity generation. These are basic pass-fail checks for a tank. What separates a "good" performance from a mere "pass" is your ability to position smartly, and to maximise your dps while still competently performing the fundamental requirements of your role.