Tank players are already rare enough, we'd just get another issue where everyone wants to queue as offtank so duty finder would be bottlenecked for main tanks, or everyone wants to be a main tank and nobody wants to be an offtank.

All you really have to do is check the other tank, if they have stance on or they pop stance then you let them main tank, if not then instead of standing around awkwardly you just pop your stance and do the pull