Yes, by default Spotify caches everything up to 10% of your total free disk space. This is space that you are not using anyway, so what is the problem? It makes playback of music you've listened to in the past a lot faster, and saves you downstream bandwidth - which in turn makes for less load on servers and the swarm.

You can change how much disk space it uses in the settings.

p2p is unrelated to (these) caching strategies. They'd do the same if it was all centralized.

Why not just download a huge bunch of songs using bit torrent and pretend to be using Spotify? No partially cached songs, no proprietary anything, etc.