Something not mentioned by Ars is that the difference in cost between datacenter bandwidth and broadband has changed significantly in the last 10 years. Datacenter resources are now so cheap that P2P has little advantage. (I think the rise of cyberlockers are another manifestation of this trend.)
P2P has an advantage of being harder to block or to throttle at the last mile ISPs.
Also, P2P generally delivers smallest connection latency as compared to all other setups.
Also, P2P makes it harder for the service provider to snoop on the traffic, and this might be just the reason why Microsoft is shifting towards supernodes. Skype is just too big of a communication infrastructure piece to let it go "unsupervised."
So while the bandwidth might be getting cheaper, there's still plenty of reasons to do things in P2P way.
