I can't speak on the MP management (though from talking to my DRK friend, it wouldn't hurt if it drained less MP in PvP) but I can say that DRK can be a really great harass character like the other tanks. You can Unleash a data point (Seal Rock) for a long time if solo you've got Grit + all your cooldowns up + living dead which makes you an amazing defender / flag spinner.

About them not having a push back skill, I'm a little bit surprised they didn't give them one like the other tanks but I think their other pvp skills (Carnal Chill for Damage Reduction and Tar Pit for CC Heavy) make up for it.