For those who didn't use Skype at the time, they had an algorithm which would promote some nodes to "super node" status. Those nodes would then be used for processing power, uploads, etc.
I guess Microsoft didn't like the possible legal implications and this "feature" went on the chopping block quite early after the acquisition (unless my memory's failing me and Skype dumped it even before the acquisition).
* The nefariousness one: So Microsoft can let NSA and the cops listen in.
* Because mobile clients don't work well with P2P.
* Because servers and bandwidth got really cheap.
"An executable-only object code form of the GI Software was licensed by Joltid to Skype, a well-known Internet-based company that provides users throughout the world with free or low-cost telephone services over the Internet. Skype did not obtain a license to the GI Software source code, however"[0]
[0] - https://techcrunch.com/2009/09/18/new-lawsuit-brings-clarity...
Yet, dissonantly enough, this is exactly the model you accept if you use proprietary drivers on your system. Arguably, it's the model you accept if you use any proprietary software - programs are not perfectly isolated from each other.
If you found this revelation about Skype to be vaguely shocking, as I did, consider if your reaction is consistent when it comes to the other things in your life - like your Android phone.
http://www.zdnet.com/article/skype-ditched-peer-to-peer-supe...
> ...the Skype peer-to-peer network architecture elected certain nodes to be "supernodes", to help maintain the index of peers as well as handle parts of the NAT [network address translation]/firewall traversal for other peers. This election algorithm chose only machines with open Internet connectivity, substantial uptime, and which were running the latest version of our peer-to-peer code.
I probably misunderstood the first part of his comment to mean that they also wanted to distribute some of their processing to clients.