Not all peer to peer applications are illegal.
An article in a publication called the Technology Review from MIT entitled Internet 6.0 says, about switching to IPv6:
But when the IPv6 rollout is finally done, not all the effects will be positive: the new Version 6 Internet will be slower, more friendly to peer-to-peer-based copyright violation systems, and the computers on it will almost certainly be less secure.
This is a strange statement. Saying that it will be “more friendly” to copyright violation systems is like saying a word processor's improved clipboard is makes the program more attractive to students plagiarizing documents. It's actually “more friendly” to every application, not just ones that are used for illegal purposes.
His point about it being less secure simply because it's new code is valid, though. As for slower, well, probably marginally so but not enough to worry about.