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.
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.
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.
