I gleefully deleted my music collection after being back in the USA and able to use Spotify. Having access to a astounding quantity of music without having to maintain a library is so, so freeing.
My biggest concern with deleting them would be forgetting about music that I previously liked. Seems like running `find` on my music directory would solve that.
Then again, with 30-40GB, I could just stick it all on a couple SD cards (for redundancy) and ignore them in a drawer.
Unsure about Spotify - but Google Music allows you to upload your own collection as well.