I too have never had any experience with or against a multi-boxer, but I cannot see the problem with them.
If you play in AV for instance you could get a group of 5 players to team up and they would (in all likelihood) very quickly kill any single people they were up against. Yes it may take a few more miliseconds to communicate that fact between themselves, but they too would beat any single opponents. This is certainly not against the rules, and no-one has any problems with it.
Multiboxing is simply an extension of this. In BGS they are taking 5 players spaces. Instead of having 5 separate players there is a multi-boxing team. There is an advantage in the ease of team targeting and no communication downtime. To this there is the disadvantage: they can easily be CC'd, 4 of their team will not react to a threat if it is near them (e.g. the whoosh of a stealthed rogue) and if they get separated or all end up facing different ways (through fear or polymorph for example) they are virtually useless.
The same goes for arena. A multi-box team is taking spaces that could be filled with people. Actual physical thinking, reacting, deciding-things-for-themselves people. The MB may have 2, 3 or 4 followers, but once again they can't react themselves, can't adapt to a situation and can't work to re-group themselves, and so I think that in the arena this will never make them all-conquoring.
I would personally love to take on a MB team. The hard thing (to me at least) is reacting to 2, 3 or 5 different personalities. The MB team may instantly kill one or two members, but by that time my remaining team members would have feared, CC'd and disrupted them, and I don't think a team of zombies would eventually stand a chance.