This is something we're debating. If suppose data was to sync with a folder on your Dropbox, we can't guarantee its integrity should you decide to remove or move it, whether accidentally or intentionally. This isn't unsolvable, but definitely something to think about.
Doesn't seem like much of a backup to me... I'd hope at the very least that even if there isn't a local storage option they would build it to support other peoples "cloud storage" products.