So basically:
This is no longer WoW in which it's hard to have multiple alts. People who play pure dps classes play them because they want to DPS. If they wanted to try out some basic tanking, which I'm sure many of them do, there is no problem doing that on another character in dungeons.
Yes, if you gave a new spec to every class, people would definitely try it at first, but that doens't mean they would suddenly start enjoying tanking. Your guess is just as good as mine, or anyone else's.