I can say almost for sure that it will never happen. If Spotify let you buy your music then you are off the grid. They actually lose you. They will lose access to the following information:
1. How often you listen to that music
2. When you listen to it
3. Where you listen to it. While driving for example?! Note that Spotify has access to your location.
4. Basically who is listening to what on the map of the world!
This information is Gold to Spotify. You can imagine what they can do with this tonnes of information. Advertising, suggesting music to me (They suggest good music!), using it as selling points to customers and so on. They know a bit too much about us. Internet or web still is a wild wild west. Every thing started by google undermining Privacy. Then facebook ruined it and other companies just followed them!
There should be a subscription for DJs allowing for the ordering of flac /320kbps versions of all the tracks in a playlist as a downloadable file. This subscription could be a little more expensive since Spotify looses some revenue (listening statistics and so on)