I think people are missing a critical part here: All content in the game can currently be PuG'd
(Note: some of the tougher achievements need a coordinated effort, but the basic RAIDS can be PuG'd!)

That being said, if I'm on a PuG for a raid, I would roll with all characters. If I'm with friends/fellow multi-boxers, then I let my friends pick their items first, and I take the scraps. Ideally, we would take turns, unless there's something specific that a person is aiming for. If a friend of mine wants/needs something specific, then by all means, I let them have it.

Often times, multiboxing is an integral part of getting through raids, especially for pick-up groups. I can't even begin to tell you the amount of people sitting in the general channel wanting to run x, but needing (usually in this order): healers, tanks, and then other classes. I see chat like this everyday on my server; I'm sure it's probably not much different on your servers.

I would agree with some of the other comments here too in stating that a multiboxer is actually often BETTER than a single player if a) the boxer knows the encounter and b) the encounter does not require LOTS of movement.

There are plenty of encounters that qualify for both points A and B in this game. Besides that, the multiboxer is playing at least 1 character at 100% capacity, and usually multi-boxed toons have similar gear (e.g. If you're running heroics, all of your characters have heroic gear)

Cheers