It's a 50/50 thing for me, some players find perks in staying AFK. As pickles_pussycat said, she likes to stay on to develop her farm, etc. But it can also free up server space for those players that want to enter a certain server but it is full, especially at times that the server is at it's busiest, which would most likely free up alot of lag space. On the other hand depending on the quality of your internet connection, most often players are disconnected after awhile of being AFK so there is a less official version of that idea. I know in Games that there are quite a few AFK players, even though you can set it to friends only I believe it still affects the server lag-wise, I could be wrong on this, not too sure. I don't think that there is too much lag involved in kicking people, as it will most likely not be kicking players all at once, but at separate times. If I am right and it does indeed affect the amount of lag on the server, I think it could come to a great convenience.