Something that hasn't been mentioned yet is the probability of multiboxing used towards an ultimate purpose of account trading. I knew people before the Moltdown who would multi-level in order to sell the accounts on third-party (fourth party? lol) websites.
This, the fact that having 5 or more accounts without paying for them is NOT Blizzlike, the clear advantage(PvP, Economy)/cancer it gives, and what Mavelea pointed out previously (more multiboxers = more queue), make it plain that it's simply for the greater good to outlaw multiboxing, for the benefit of the majority of the Lordaeron playerbase.
The selfish gain it gives a small group of people is simply not worth the ire it causes for others.