The problem that I have is that it's not clear to me why you shouldn't look at it by comparing it to CD sales. The goal of Spotify isn't to capture all the pirates and leave the rest of the public paying for their music -- it's to replace other forms of music consumption.
In one sense you could compare Spotify to radio and say that it's no different to that. But it is different. On radio, I can't select any song I want to listen to. On radio, I have to listen to what's on a playlist someone else has created (often in a manner that avoids repeating a particular song). Radio doesn't replace the needs to buy a song in order to listen to it repeatedly. Spotify does. And if it's going to do that, I think it behooves them to pay more to artists (or to place some kind of restriction on the number of times a song can be played).
In one sense you could compare Spotify to radio and say that it's no different to that. But it is different. On radio, I can't select any song I want to listen to. On radio, I have to listen to what's on a playlist someone else has created (often in a manner that avoids repeating a particular song). Radio doesn't replace the needs to buy a song in order to listen to it repeatedly. Spotify does. And if it's going to do that, I think it behooves them to pay more to artists (or to place some kind of restriction on the number of times a song can be played).