Google music makes a decent music storage locker, but it doesn't separate music you've uploaded from music you've purchased, and I see no way of storing music you've purchased locally (for instance, what if I'm going on vacation and am going to be stuck in a plane for 3 hours, it'd be nice to take my jams with me!). Most other services such as iTunes sync to the device, so there's no problem there.
So basically, you 'buy' music that you can't use without an online connection, and if you are going to do that you might as well go with a cheaper more user friendly option like Pandora/Spotify/Last.FM.