I agree. If it had an option to be polite about using upload bandwidth (e.g., do a multicast advertisement to find the count of total Spotify clients and divide

I agree. If it had an option to be polite about using upload bandwidth (e.g., do a multicast advertisement to find the count of total Spotify clients and divide upload bandwidth usage by that number) and could easily be sandboxed such that you know it isn't scouring your whole filesystem for stuff to upload, I would be okay with it. For example, it (at least last time I looked on OS X) goes poking through ~/Music/iTunes/ presumably to find music files it does not have to download. That's a little too far.