EDIT: Okay, I got the first part of Nikodemus's post, I think. Doing nothing, is still taking a spot on the server, that can only handle 200 people at a time. You don't need to use bandwith just parking your character outside harnquists and going to work. The server doesn't have problems if there aren't too many people logged on, but it will if it reaches 196 and 20 have been AFK for 4 hours. Also, the game has a problem with it. It's not nice seeing people standing around for hours at a time, not moving a muscle. I haven't figured out the rest yet.
Sorry, i gues i complicate things without a reason and so i make errors.
What is wrong in taking a spot? It is not the spot which is causing problems to a server, but it is the bandwith. We can't really say, but if it is 1/20 of what would be for an active player, then where is the waste? If you throw away all the afkers, you will get 20*1/20 rooms for active players, which is 1! wow!
Considering the theory that from the 20% you talk about, half will be afk for no longer than 20 minutes, you are getting 1 room for one active player every 200 players. Really worth the efford.
But is there oneone who knows how much afker is really using the bandwith?