I guess this will be downvoted, but still: - If the p2p feature was opt-in, no-one would have enabled it; but the technology needs most users to have it enable

I guess this will be downvoted, but still: - If the p2p feature was opt-in, no-one would have enabled it; but the technology needs most users to have it enabled to operate optimally. And I don't see how it could be done without enabling it by default. - Since (mostly) everyone has it enabled by default, no peer will have a disproportionate amount of upload traffic. - It makes sense to download a Service Pack (read: any big update) once and have it sent to all PCs on the LAN automatically. - It's not enabled on metered connections and won't compete for bandwidth with other apps. It can be switched to operate only within LAN. It can be disabled forever.