If you're dealing on a small scale (e.g. "Hey, icebraining, let me grab that file from you!"), it's going to be easier to just send a share request from your Dropbox account to mine, which has the same end result (flipping an ownership bit in Dropbox's system).

That doesn't really scale, no, but sharing hashes doesn't really either, unless you want to spend a significant amount of time building infrastructure to keep track of who has what files and automate the management of sharing hashes.