As success in this game is largely achieved democratically a multi-user would be afforded much potential power by his multiple characters. If the multi-user were to exercise his power he could win positions for himself and, by so doing, deny those positions from other players.
There is also the matter that bypassing the character limit with multiple accounts defeats one of the incentives for donation, and thereby reduces potential profit for the owner. Of course, in regards to Battlemaster this sort of thing is negligible, as there are few cheaters and few donators to make it substantial, but for other games it could be a big deal. I'm sure that Valve is more interested in banning multi-users in TF2 due to the potential lost profits, rather than the unfair trading advantage they have over fellow hat traders.
