[Keith]

One interesting suggestion I heard is that music sharing is people's way of routing around the stranglehold that the commercial hit machine has over radio.

The RIAA like to count each song pirated as lost revenue, but in practice:
(a) the vast bulk of people who pirate a song never would have paid for it anyway; and
(b) they make sales that they never would have if not for piracy! ie. People are using filesharing networks to listen to songs and see if they can justify shelling out for the album.

It's also an end-around for how difficult companies make it to take a CD that you legitimately own and copy it to your MP3 player.