There's a growing voice in the buzmill that the new cloud music lockers are going to make downloading music an anachronism.
Particularly for the rumored Apple offering, which (supposedly) will see what tracks you've purchased and automatically move them into your locker so that you don't need to upload, this seems attractive (Neither Google or Amazon have the agreements in place with the labels to provide this yet).
From a top-down perspective, this sort of thing makes sense as well: if you have track (A) in your locker as does about a million other people, why shouldn't there be a single instance of that song and simply an indexed pointer in your locker? Then the services could house LOTS more music and even better, if the artist/label/service provider whatever gets a 256b version rather than 128b, instantly, everyone gets an upgrade. But how many people want to be network-separated from their music? How many times have you gone into a building and suddenly lost 3G?

Additionally, there's the startup stream time - Apple has a patent for keeping the beginnings of songs on your device so that you get instant start, but who knows? There's the upside, which is that your car could now stream your library, but we can do that with our smartphones and an adapter right now.
I'm just really curious if you're interesting in the cloud locker. I *think* I am (particularly because of backups and not chasing drives) but am looking for the killer reason.