And that's really where the issue lies -- most of these people aren't no-lifers, they're just folks with money to burn that want to trounce a battleground all by themselves or run Karazhan solo. But at the same time, there is a sense of superiority that comes across -- they brag (under their breath in a crowded restaurant) that they can beat anyone else's player, and they claim that "other people can only see a quarter of the game," which just patently isn't true. This is a generalization I'm making, of course, but playing 10 characters by yourself seems to give an inflated sense of self-worth in the game (after all, aren't you 10 times better than someone playing with just one character?) and that could be the reason that multiboxers have the reputation they do.
Let's break up that generalization a bit: not everyone who multiboxes spends tons of money on it (some people use a spouse's or other family member's account), and obviously not everyone who multiboxes is as smug as the people at this dinner sound. Some people do it just to mix up the game a bit, and there's nothing wrong with that. The second you start thinking, however, that you're a better player than someone else just because you run five accounts by yourself, then you're off into "inflated sense of self-worth" territory.
But for better or worse, Blizzard has approved this behavior -- if you have the money to spend on multiple accounts, there's apparently no limit as to how many you can have. Just like twinking, players who choose to play the game as it's traditionally designed may not like it much, but with Blizzard OK with it, multiboxing is here to stay.