Makes perfect sense to me if you don't trust your host not to report you. Keys are small enough to just send in a private chat rather than hosted alongside the encrypted file.
https://tahoe-lafs.org/trac/tahoe-lafs
(One of the properties of Tahoe is managing the keys to decrypt data completely outside of the servers, so that the servers never have any knowledge of what they're storing.)
Don't do illegal shit on Dropbox; problem solved.