DA doesn't put you into Demon form. It gives your robe wearing warlock wings. Just like everything else, something new will always be rooted for. But it would fade. Of course you would be tanking. Why wouldn't you? But after the newness of it wore off, and ppl realize "oh, now I realize why I didn't roll a tank in the first place.". It'll be wasted resource time. It just isn't worth the immense amount of work that it would take.
---------- Post added 2013-06-19 at 05:47 AM ----------
And most ppl don't realize that if they want to play a hybrid, they can. There is nothing stopping them. At all.