If 20% of your network goes down and you can still serve clients normally, it means that you have a big reserve of machines useful only in case in big outages. I don't know if you can justify it economically.
Most existing companies don't run P2P voice chat networks, either. Using EC2 or some other elastic cloud for emergency supernodes makes a lot of sense, since they can outsource the risk of those machines sitting idle to Amazon.
That's an interesting thought: in case of outage Skype could switch from user supplied resources (Supernodes eating users bandwidth and processing) to emergency Skype hosted supernode services.
