If 10,000 people buy a track from iTunes, I can pay off my credit cards and keep making music. If 10,000 people listen to a track on Spotify, I earn $13 and change.
Spotify may have spent all their effort on the US launch to woo the big labels, but I’m more concerned about how much of an impact this will have on smaller bands and individuals. I doubt U2 would notice the impact of Spotify either way, but fifty album sales can have a noticeable effect on the indie musicians. Support good music, don’t just stream Spotify forever, go buy the album!