I think it would be reasonable and add some more depth to the game at a certain level if this was possible.
I chatted with a few of the DEV's about it at the last blizzcon and it has been internally discussed as "Traitor" role playing and so forth is evident in the storylines yet not possible for players.
Be cool if you could be able to do a nice quest on your race, to defect.
The biggest reason it would be challenging at this stage in the life cycle of WOW is the amount of QA that would be required for what would be a novelty feature at this point, so many retests of the quests/paths required and this thing would appeal to a limited audience.
Did I just solution myself out of this change by identifying the benefits don't exceed the work effort? ROFL I must be getting better at my real life job