Labels use Youtube to promote their stuff. Even "He who must not be named" does so. If they want they can request Youtube to remove it and away it goes. if you send an MP3 to ten friends and they send it to another ten you soon have a bunch of people who can burn the track to CD, sling it on their iPd and so on. The label can't come round to your house and take it off your harddrive.
All those people who have a good quality MP3 are not likely to buy the track. Those who can access a low quality version on Youtube are a bit more likely too. That's the logic anyway