Sharing a Dropbox folder with Host & Guest OS (Virtual Box)

I recently tried the same thing, and my conclusion is the like the one you describe in your edit. I did get a lot of revisions quickly when having Dropbox installed on both the host as on the guest. This will show up in the version history of files that were accessed by both machines at the same time (I happened to be testing the Linux and Windows version of an application concurrently), so there was a ping-pong of "Modified on machine Host" and "Modified on machine Guest" messages. All these versions aren't really an issue of course, but it also isn't a pretty thing to do (both on your own network connection, as to Dropbox' servers).

So as you said as well: when the host has Dropbox installed and running, it will detect all changes you do in the guest, as it is the very same data that is being modified. The issue is made more clear if you would have a Windows guest: in that case, the shared folders will show up as a networked/shared directory. Using Dropbox on that folder from the guest in that case would've likely raised some eyebrows...