If used properly I really don't see any problem with them, but I still think that they are a negative thing because as a musician I would not want my work stolen and pirated. Then again, that's just my opinion. As a songwriter, a computer nut, and someone who is a lover of music, it's a two way street. It also hurts the musician, the companies they belong to, and in the long run, it hurts the consumer. I'm only for MP3, if it involves songs that are specifically released to MP3.
People can use it as they see fit.
If you have a good song, it'll make money one way or another, trust me. So let it be. No, so don't worry about it. About time we have MP3 players in our cars, you won't be able to write music anymore anyway. As a musician I can showcase songs in excellent quality over the web.