B ) If you could do this, you'd be lost. Do you really want to suddenly have a lvl 80 Guardians when you've never played a guardian before. Do you think you'd have any idea how your skills work or be a good guardian?
If you want this, go make an Alt that looks exactly the same as your existing character.
I'd say point B is a fallacy, as you can already have an instant 80 in the game via sPvP. Learning how to play a new profession/build isn't difficult, it's all a matter of how much effort you put into researching your choices. It's fairly safe to assume that a fair number of those wanting to pay for a profession change are doing so because they are pretty well informed of how that prof plays. It's just a matter of getting through the 80 levels before your character is fully functional outside of sPvP.
I'd be willing to wager that they would make more money off race/prof changes than alt slots. I also feel some of the technical difficulties behind it, specifically personal story, would be the lynchpin that prevents them from doing this.