I think the answer you are looking for is. The server would crash. If you consider how many multiboxers post on this site alone, that is not counting the people who read this site and don't post. The sheer number of accounts that would be active and trying to log in together would cause a huge que/server crash due to over population. If blizzard made say US-Illian a boxer only server then it might work.

On a side note DEATH TO THE ALLIANCE!!! Horde 4 life.