Of course, Dropbox can't prevent people from sharing content out of band. But if Client A and Client B are offering arbitrary byte ranges to complete strangers, they are effectively playing BitTorrent again.
Yes, but they are only exchanging a constant amount of information to fool the server challenge, whereas we could hope to do better if the server builds challenges which use information that he knows the client has.
For some reason, this inspired me to write a blog post: http://a3nm.net/blog/deduplication_attacks.html and http://news.ycombinator.com/item?id=2489594
Client A wants the file that Client B has so when Dropbox asks Client A for some random offset, Client A asks Client B in the background and relays the result to Dropbox.
It really depends on how far pirates would be willing to go.