I do believe that most of the people here, reflecting against multiboxers are talking about the particular type of them:
The pew pew 5x single class glass cannon teams: 5x BM/MM Hunter, 5x ELE Shammy, 5x Blood/Unholy DK, 5x Boomkins. Anything other than those are not viable in PvP because of skill curve. Believe me.
That's like 10% of total multiboxers in general.
How come?
Most multiboxers are more interested in the challenge of world PvP and PvE in instances.
"But PvE is boring...?!"
Well, it might be boring, if you are playing solo and you have an brain-dead party with you, low DPS, bad heals, aggro loss, etc.
As a multiboxer, this is a challenge. You control everything: Tanking, DPS, Healing. It's not as easy as just pressing a single button and hoping that you kill Normal Mode bosses, not talking about Heroic Mode bosses. In theory it sounds just like that, by looking at this topic.
In reality:
If you are a normal PvE boxer you will build a team which is 5 separate classes, which share no loot if possible:
Warrior/Paladin+Hunter+Shaman+Priest+Mage/Druid is the most common setup.
To drive such a setup, one must have at least a mouse like Razer Naga or Logitech G12 and a keyboard with at least 5 extra keys somewhere.
Personally, to handle such a team it's required to use at least 48 keys, just to perform. Don't forget keys required to use different heals on specific characters, interruptions, etc. Imagine using that in PvP scenario with 5 different classes, 5 different skill sets, tons of independent AoEs, instant casts, rotations. Using AoEs sometimes tend to be even impossible in some scenarios, so most boxers cannot even AoE, not including the Starfalls and Chain Lightnings. It might get your head spinning just reading this to be honest. :)
So if you want to have the lowest impact of boxers, let us do the challenge which is dungeons and world PvP. World PvP is the most entertaining, trying to fight people's guilds, getting killed, trying to resurrect all 5 chars separately, it's fun, even if I get killed 10 times, but I can still get one of your friends/guild mates or even you, I am quite entertained.
BG/Arena PvP is for the other kind of boxers, that is all.
The most important point:
No multiboxer will EVER attack you first. It's that plain simple. You fight - we fight back. It's in the unwritten codex of multiboxers. You wave - we wave. You dance? We dance. Only possible cases of attack would be equally scaled teams, like 3 of you, but with way higher levels, 5 people or more (cities, etc) or just plainly silly, you going one versus my team, I really cannot help you beating on my healer, that is all.
Peace out.