Unless I'm mistaken I think there's something else going on here that may be getting lost in this copyright noise. What if instead of looking at this as a method for illegally trafficking in copyrighted material one looks at more like a backup system?
If I take my legally owned MP3's and place them into OFF as a safeguard against my machine crashing and losing that data am I violating a copyright? Assume that I don't share the magic URLs. Haven't I then just simply made one copy of the file that is now stored in the cloud?
Now my laptop falls off the back of my motorcycle on the way home from the office. It goes bouncing down the freeway finally getting hammered by a bus. Total loss. Can't I now retrieve all my MP3s by simply accessing the URLs that I created (and hopefully stored somewhere else!).
Have I reduced my personal backup storage requirements by keeping URLs instead of the original data? Have I increased my chances of getting my data back because it is no longer stored in a finite number of places ( 10 ) that are arguably less geographically dispersed than the OFF system?