It depends. If I already have the full file, it's much easier (and efficient) to tell you the value of a random byte and let DB give you the file than having to transfer it all.

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.

Of course, but then you still need infrastructure to figure out who else has the file, how to keep things private, etc. You can't just distribute a hash and be done with it.

I didn't miss that part, I was simply saying that a client that had the file could tell what byte 1257 was to the other client (or some other random byte, as requested by dropbox).