I think the key point is to use the right tool for the job. I use Dropbox only for files that I want to be available on every computer onto which I install it. While one could probably coerce Dropbox into a different model of use, keeping track of which computer has which files is more work than I would want to do.

There are other services that one can use for cloud storage instead of cloud sync. Your wife should consider simultaneously using cloud sync (Dropbox) for the files she wants to be constantly available everywhere and use cloud storage (such as Box) for files she wants to be available for download.