I wish I'd been invited to this at BlizzCon -- TGDaily sat down with a group of multiboxers last weekend, and lived to tell the tale. I don't think multiboxers are the most hated people in the game (Paladins and Warlocks probably get to trade off that title), but it's true that just like twinks, they can draw some ire. Blizzard has given them a free pass in the game (most likely because they're spending ungodly amounts of money keeping up their fleets of characters), but apparently multiboxers are just people. People willing to shell out lots of money and time on overpowering themselves inside a game.
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.