I myself never tank, I just don't whatever is needed to even have a desire to tank. With that said, anytime a tank spec\class has an advantage doing something, I'll go tank. When it was Blood DKs soloing the troll warbringers, I was there. Now in Legion I found it's just easier to do abut everything has Prot spec. My first 2 withered training sessions ended with me dying in the first room on the first pull of the arcane guy. Rolled Prot and now I'm superman.
But you're never going to see me tank a dungeon and of the others that I know who have never tank, but switch to tank spec to quest in legion, none have plans to tank more than a World Quest boss. But Blizzard's plan of making tank spec the better way to play might pay out more, though I suspect it will be like it was in the heavy DK days, in which you have a bunch of people who try to tank but have no idea what they are doing.