In the upcoming releases, desktop versions will have full access to your cloud file system. But it won't be a file-sync service like Dropbox. It will be a system where the desktop can directly access your cloud store, including the ability to browse, open, and save files. From the perspective of Mathematica, it will be like saving to an external file system, albeit with a modified system for file browsing.
We're definitely thinking about whether file-syncing should be on our product roadmap, and we'll be listening to our customers to hear what they have to say on the subject.