It's still silly to upload all the music I have that they don't when they probably do already store it for someone else. Just checksum my files to ~prove that I have it. I'm sure no one cares enough about it to generate hash collisions for MP3s. Do they really store unique copies of every mp3 for ever user? That seems insanely inefficient, and yet I wouldn't be surprised if their contracts with the music industry stipulated that they couldn't share files between users.