The advantage of open networks for file sharing is that the amount of files available is simply huge, and nearly everything anyone wants is available all the time. A network like Kaaza probably represents one of the largest and most complete music libraries in the world. This is because of the so-called "Network Effect": the value of a network is proportional to the square of the number of users.
Private networks don't have that advantage, because users only share among their friends. If you want the new Dixie Chicks single, you have to wait until one of your friends rips it from their collection.
What the RIAA is trying to do is scare people off centralized networks so that their network effect is diminished. To see if this is working, you would have to find statistics about how the number of files shared has changed since the lawsuits began. If the RIAA is successful at knocking people off the open networks, they will become less valuable and I think then we will see people starting to move to semi-private networks.
In many ways, this would be a return to the pre-Napster status quo, where people got their filez from private FTP servers, internal filesharing, IRC channels and password protected web sites.