Georgia Tech/PARC research says that sharing music builds community and that users judge others by their playlists:

When one user decided to share his music, he recalled: “I just went through it to see if there was not like stuff that would be like, I don’t know, annoying, that I would not like people to know that I had.” Sensing that his library was “not very cool,” he added more music to create a “balanced” portrayal of himself.

Another participant was worried about what his co-workers would think of the Justin Timberlake and Michael McDonald music he had purchased for his wife and included in his library. Yet another user crafted his library around his German nationality and collection of German band music he thought others wouldn’t have. Meanwhile, other users hid their expertise because they thought their co-workers would not relate to it or find it distasteful.

...via Copyfight