You're paying for each account, so you have a right to bring in as many as you'd like.

Played properly, a multiboxer won't hurt the team and can be a considerable help. It's like any pre-formed group, there are good ones and bad ones. A bad multiboxer can be like a bad PUG.

In my opinion, you are most likely to make friends in BGs if they see your worth. In EOTS and AB, I have more success at this when I'm leading the offense. Sometimes this means leaving a node completely undefended, but it can look bad to see 4 people just sitting at a node or tower doing nothing at all.

In EOTS, if you have 4 toons in there, then it's important that others not follow you. A solid winning strategy is to control 3 towers and spend little time trying to get the flag. If your 4 can control a tower, that leaves the remaining 11 players to control the other two towers which is a perfectly reasonable division of labor. Sometimes you have to be a little proactive and remind people not to gawk and follow you around, having six people sit at a tower is likely hurting your team.

In AB, the math is close to the same, and I think a multiboxer generally is doing a service if he can take and hold Blacksmith. That's been my strategy, there is usually a fair amount of action there and I generally don't lose it (with a 4 shaman team) unless I'm facing a very organized group of 4 or more people.

WSG is more iffy, in my opinion, if you are bringing in four or more toons. Played correctly and with good communication, a multiboxer can be an asset, but it may require some additional communication if you are playing against a competent PUG.