The biggest problem with the whole "flip DPS to tank mode" thing is that it takes a very different mindset to tank then to DPS. As a tank I have to plan the pull enough as to not pull more mobs then intended, look after the healer and control the mob's behavior. As DPS I look out for my own health and keep an eye on the healer to see if they need help. Groups are very brutal on tank errors and I have not seen many DPS that have a thick enough hide to shrug it off. I do not believe that enabling other classes to tank would produce more tanks, imho tanking has more to do with the player mindset.