I have tracks and albums on my PC which aren't currently (and probably never will be, like Nujabes) on Spotify. I still listen to them on Spotify through the local media folder, but it would be so very nice to be able to sync my personal music with you guys, put it up on your servers, and then sync it to my phone.
I mean that's the point of Spotify right - to have all your music in one place. Plus their are loads of songs I can't listen to on Spotify in my country, but own the CD of. Allowing cloud-storage of those kind of songs would sort that out.