Crits happen all the time, there are just fewer fights where the bosses have VERY high crit rates (Shiva Ex). It's still a great cooldown for tankyness, cause if it even prevents 2 crits in its duration, its helped 'evened' your damage for healers.

VS Mercy stroke is something a DRK might use for soloing since with your cd, it might be used once...MAYBE twice in a raid?