So I'm assuming you are smartish and don't want the GOOD dps/heals to swap to tank. That makes no sense. But if someone is bad at heal/dps can you trust them to tank well? Nope. SO who gets to tank then? The people who already are... Those who are bad and are too slow to realize that they are bad, Those that are good and enjoy the class. The mounts will change nothing. Tanking is just not as popular as healing or DPS because it is somewhat a thankless role (though my recommendations somewhat disagree...), it doesn't have meters or tangible records of how well they are doing. So people don't like it because it's not really self validating all that easy. Mount... won't help.