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.

There are plenty of valid fair use reasons for me to share copyrighted material.

Example, my wife and I are physically separated (let's say she is in Europe for a week) and she wants a copy of "Frozen" that I just ripped from the DVD I own. (It's a long plane ride home.)

Making a copy is fair use, and putting a copy on her computer is not illegal.

Dropbox is treading a fine line. If they fuck with my files, I will drop them fast.

You're right that that's what traitors say about sharing but that's not a real world solution for serious people. Perhaps you aren't aware of this but copyright holding monopolies have purchased laws in the US that criminalize perfectly normal behavior, like sharing copyrighted material over a computer network. My friend, remember that we can't allow these traitors to wreck our brains with propaganda and make us think that publishers deserve a payment every time someone even thinks about a song or movie.