The only real downside to buyable 90's is that you don't have the leveling process with which to learn your class mechanics and abilities.
However, can you honestly tell me that you think a new player would be able to learn, given the current leveling process? I sure don't think so. There's also the point of playing with your friends. New players now-a-days are generally brought to the game by a friend that's been playing for a while. However, most people will then have to wait quite a long time before they level far enough and gear enough to legitimately play with those friends. Not to mention having to learn their characters once they reach level 90 to a sufficient skill-level that their friend wouldn't need to carry them through the majority of content.
WoW is an old game. Buyable 90's don't hurt anyone.