As a side note, it amuses me to see this repeated so often, given that it's not true. If you do the math, Spotify pays about the same as a radio play. You just have to remember that each radio play goes out to a few hundred thousand to a few million people each time.
The reason artists don't like Spotify is not because they pay poorly, it's because they pay fairly. When you get a radio play, it's assumed that all the people the station reaches listened to the song. Spotify knows exactly how many people listened to a song, and pay accordingly. Unpopular artists don't like this because they get accurate counts.