A lot of people join a Network to get those traditional features such as banners and whatnot. YouTube changed the Partner Program on April 12, 2012, any Partners after that date have no way to get those features. So the only way now is to join a Network but YouTube want to push to get the extra features available to every Partner that will probably be a while. Other reasons that people may join a Network is higher CPMs (not guaranteed), support, production support (example RWJ filmed at Maker), promotion (some Networks) That's all I can come up with in a few minutes
Check this out
http://www.youtube.com/yt/creators/join-mcn.html
Stick with YouTube if you're a pre-April partner, Freedom!