Yeah, in some fight where all you do is stand there and shoot at easy-to-kill mobs, you can kill twice or four times as fast - but what about when those mobs start attacking you? You then have to learn how to move those characters independent of each other in efficient ways.
Real Arena players in WoW can tell you about multibox teams.They have their serious strengths, but they have their serious weaknesses, too. Generally, a good multiboxer can kick hiney on any weak Arena team with poor coordination, but an experienced team that can spot a multiboxer knows how to crush him.
As far as multiboxing in D3 - I'm really at a loss as to why anyone would care. There is no one else in a 4 person multibox co-op to effect in any way. And, if you get someone dual boxing in your co-op, you can just leave, or ask him to leave. Most multiboxers go out of their way to be polite. They know they are controversial with some people, and don't want to make it unpleasant for themselves and others. As far as I can see, you're just jealous for some reason - maybe you can't afford 2 or 4 copies of the game - and trying to rain on someone elses parade because you can't do what they can.
Whatever the case, you should get over it; multiboxing is allowed in most games that involve more than solo play. It's here to stay - and, it has almost no effect on you. It's actually too hard for most people to learn how to do right, they just don't want to take the time to develop the skills needed. (Not to mention set up the software properly. Even with pre-made multiboxing software, it's not a 5-minute job, unlike what some of them will tell you.)
I don't understand why we can't just all be friends here and let everybody play like they want to play. I mean, maybe I wouldn't like to play co-op with you because of something you do, or vice versa, but hey, that's cool, we can just find different people to friend that play like us, there's no sense me hating on you because you like to play different and trying to stomp all over your play style and stir up people against you.