How cool would it not be to be able to join more factions?
You could play Ally and ex. join a faction that was join-able by horde at the same time.
Or factions that both was against Ally and Horde.
This all with sub races could be pretty sweet and open up more and interesting gameplay and story modes.
I for one would like to be in a faction that causes chaos and mayhem, not everyone wants to be a Hero in his shiny thingies who always fights the same villains as the opposing faction, a bit boring if you ask me.