Amazon just launched their own music service. I can easily see someone like Microsoft trying to jump in and buy Spotify to create their own version.
Rhapsody was the only competitor at the time that I knew of. The Zune Pass service didn't really catch on because, well... you needed a Zune.
They had in-house versions of everything Skype offers, yet they spent $8.5 billion on Skype. They had an in-house hardware team, yet they spent $7.2 billion on Nokia Mobile Devices.
If I had to guess, Microsoft is probably considering a Spotify purchase. The Xbox brand is ailing recently, and they must be getting a bit scared.