Multi-boxing is allowed for several reasons. First, it isn't cheating in any way, as there isn't any item or spell exploits, or packet editing going on. Secondly, Blizzard allows this, therefore it is "Blizz-like". And last, there is nothing a multi-boxer can do that a group of the same number of opposing characters cannot. Anyone is allowed, also. Which means you are more than welcome to retaliate against multi-boxers by multi-boxing against them. It raises challenge, competition, and fun to certain aspects of the game, for both multi-boxers and their opponents, where there would normally be none.