Think about it.

A big company is gonna pay 100000 bucks to have their commercial aired on MTV twice a night for 12 seconds, during a week. Now, everone knows you don't really listen to those ads...

Now, the company pays 0.25 bucks for each download of the song. 15 cents goes to the host, or internet server, 10 cents goes to the artist. if you get downloaded 1000 times (it's free, why not?) you get, as an artist, a 100 bucks... And the company has touched a 1000 kids that will think their ad is cooler, since the group that is after the ad is quite cool... Of course, you can take it off, but the ad could be a 1 time only thing. And by the way, for 1000 downlads, you pay only 250 dollars, so with 100000 dollars, you get at least 400 000 downlads, so 400 000 people that will listen to your ad, with a public that you know (young, hip etc...)
I think it's quite a good deal... Well, a 100 bucks is not much, but it's a 100 bucks more than you'll get in your room playing those jason Becker stuff... :D