It was just a subjective question. I thought Dropbox could just extend this functionality.

Or maybe 3rd party apps/services could felicitate this just the way e.g. http://calepin.co does for static site generation. Use Dropbox for storage of emails. And naturally you'll have a local email archive on all your connected devices.

So, seems there's nth like this as Dropbox doesn't let us to do the compounding, as you also pointed out.