Being a multiboxer here is what I think of boxing in BGs:

I think its clearly unfair.

The arguement, "5 Boxers is no different than 5 seperate people," is weak at best. The targeting/casting time of a multiboxer is vastly faster than 5 indivisual players.

Now let me also say, boxers in BGs dont bother me personally, so on a personal level I could care less. However, from a technical standpoint, yes I think boxers have a very distinct advantage in PvP.