So how is "owning" a CD any different from Spotify?
Also, the value of a song diminishes the more you listen to it. Seriously, even if you love a song, put it on repeat for a day and you probably won't listen to it again for a couple of months.
So it's not like Spotify customers will one day buy the music, unless they really have very specific needs, like being somewhere with a bad or expensive Internet connection, but if you subscribed to Spotify in the first place then I doubt that.