Despite being on the "Official" Multi-boxer server, I've only run into multi-boxers THREE times as a Multi-boxer myself; once pre-multiboxing.

1.: Ebola or Zseth I think, in IF on the AH roof dotting people from above(and who says they have 3 lv20ish locks logged out on the roof to summon them back at any time, and I'll still report your ass x3 for roofing in Azeroth if you do try it ;p Don't break rules as a multiboxer yo)
2.: Dondiego, at Jintha'Alor; I was getting my ZF hammer and saving Sharpbeak while I was up there, I had just stealthed and moved up a tier cuz I didn't feel like clearing the mobs for once...about a minute later I see red dots show up on my minimap, I hover over and bam. "Dondiego Donmarvolo etc"; I go "OH SHIT" and hide on a ledge hopefully with my lowbie stealthed alts out of view range just before he rides by on his 5 epic skele horses, doesn't spot me...comes back down a few mins later and I narrowly avoid running into him again, I wasn't about to risk a fight when I just wanted to get my hammer and gtfo lol
3.: Couple days ago in hellfire penn, helpin a lowbie druid being ganked by random horde, he spots a multiboxing horde group near Zeth'gor; I ride over and sure enough it's a Zergling ;p I jump him with the druid, go back to killing buzzards/looting zepp parts, then go back and kill him one more time for pvp vid footage since that's why I logged on in the first place(was questing till horde took the towers, we had all 3)

You'd think I'd have run into them more often being on the "official" server with them bragging about their member count being 500+ now lol