I never use the whole "doesn't make sence with lore" argument, given that non-magical warriors somehow turn into giant chocolates, throw bolts of lightning, absorb damage, and jump 100 feet.
So I do think locks should tank. Next expac.
Pfft, that's because we're so awesome we can do that without magic.
C'mon, I thought you knew better Failz :P
Honestly though, more tank classes won't solve the tank shortage. Been done to death, still 100% true. We have five tank classes compared to the three I remember seeing when I started playing in Burning Crusade, and the demand for tanks hasn't risen any more than it did when LFD was first enabled. And even then, four tank classes did the job just fine. It's the perceived notion that tanking is hard and unforgiving (neither of which are honestly true), and that you're not in a role where you can just sort of sit around and coast along while doing your job (only partially true) that creates an artificially small amount of tanks. That's even before we get into how many raid spots are available for tanks these days (hint: not many).
Right now, we have five classes capable of tanking, and nothing has changed. This should be a very easily understood sign that the playerbase obviously doesn't want to tank on the whole, outside of a minority of people (as compared to healers or DPSers).
Making warlocks the sixth tank class, in addition to creating a host of balancing issues for the spec (prepare to get nerfed, because you will. Can't have your cake and eat it too), will not solve the fundamentally underlying issue; that it fixes nothing.
But then again, we've all talked circles down this road already and no one seems to be listening anymore, so; go ahead, keep clamoring for Blizzard to make warlocks the next tank in whatever the hell the next expansion is: I don't see you guys getting to be one without them fundamentally altering your class down to the foundation.