Next to no artists own the rights to their music. So, there will be no significant catalog on this service unless people falsely claim to be the rights holder (as they do on Grooveshark) and upload the content under DMCA, then start collecting fraudulent checks for the material.
But the main problem is, the kinds of people who will install garbage on their computer in order to get free music are already getting it through Pirate Bay, etc. If I care enough about getting it "legally" through this service, wouldn't I have a problem ripping off the other companies whose sites I browse that depend on the ad revenue I'm now depriving them of? "Hey music lovers - make sure your favorite artists get some hard earned money from their music by taking it away from all those writers, game makers and other people whose products you use!"