I don't know the real reason but for myself, I actually uninstalled Spotify after I found out about the P2P feature. There was a noticeable lag when I would play online console games and had Spotify open on a PC or laptop. It was never really an advertised feature and it kind of pissed me off that they would do that without a clear notification. Took me a bit to troubleshoot as well. I'm sure it was in the user agreement but come on, who reads that.
It was removed in 2014 in favor of central distribution.