Not all P2P is the same.
Supernodes--what's wrong with them?
In using an application that uses supernodes, like Skype or Kazaa, you open your computer up for other users to use your resources--including your memory, cpu and network bandwidth. So without you even talking to or sharing files with anyone else, other people's communications can be relayed through your computer, using it for their communications. This results in a slower computing experience for the user, less internet bandwidth for the user, and a potential security threat as the supernodes have allowed access by these external parties.
Supernodes--what are they?
In some P2P products, like Skype and Kazaa, they require the use of intermediate computers between the source and destination computers (the peers). So, for example, if Anna logs on to Skype, she can become a supernode. Let's say that Bob and John log on to Skype, and Anna is logically between them. When Bob contacts John it will most likely be sent to Anna first, then relayed on to John--and vice versa for John to Bob communications. In this way, Skype can spread out its network load across its user base. This also means that Anna's computing resources are being used for Bob and John's communications, which results in slowing down Anna's computer, using more of Anna's memory and using Anna's internet connection--a slower user experience.
Supernodes--How do I get rid of them?
The only way to get rid of the supernode problem is to remove the application and stop using those services that use supernodes. In the case of Skype and Kazaa, if you do remove them, be sure to verify that all the background programs have also been removed, or they may still run and you may still become a supernode even when you don't want to.
Source:
http://www.raketu.com/en/no-supernodes.php