Your missing the point...
Pointing SW's workspace to a Dropbox folder would use your account frequently. If you also use SW frequently, of course (and i suspect you do at least every 90 days).
Also, the point of SW + Dropbox isn't so much for backups but, in my case, it allows me to work with the same workspace on any computer. It's like a cheap and automatic SVN, without the revisioning part.