Although I'm currently taking a break from mutliboxing, the #1 reason I get hate messages is because of taking up too many valuable BG slots with my 4 shaman. Either I would completely dominate, or I would be utterly useless and I would die so much that my toons would end up in separate graveyards.

Multiboxing in BG's seems to be mostly psychological warfare. Either they're afraid of you and their ranks become disorganized and picked off individually, or there's one blood elf with half a brain who rushes in and silences your group and the rest of the team disables and CC's you. After this first kill, they aren't afraid of you anymore and it goes downhill. Or maybe I'm just bad.