"Just play something else" will NEVER EVER be an acceptable response to any issues in any game. People want to play DRK, and they want to enjoy it. The class MUST be good and well made. Why should a job even exist if everyone who wants to enjoy it would have more fun literally not playing it? This shows a profound lack in understanding of how game balance work. Not all balance is numbers. Playing EVERYTHING must be enjoyable or why even have it?
The problem with Dark Missionary is that it already exists 2 more times as of EW. RDM gets it and GNB has it. If multiple classes have the same thing, one will do it better than the other, and in this case, why even need it on a tank if you have a RDM? Pair it with tank shielding and have the RDM pop it and the DRK will sit there twiddling his thumbs because stacking defensive cooldowns is inefficient. DRK doesn't need something better it needs something unique that is universally useful since all tanks have that except GNB which makes up for it with it's ability to go full doomslayer and rip and tear their DPS to compensate for being 3 DPS in a suit of platemail.