I think the idea is pretty interesting (at least on the surface), but after trying it a few times I wonder what the utility of it will be in practice. In particular, I'm trying to think of use cases where I would have access to e-mail, but wouldn't have access to either the Dropbox application itself or the Dropbox web interface.

And, as others have mentioned, the whole point of Dropbox for me was to avoid having to send file attachments in the first place.