> You can also consider Dropbox a simple, automatic backup system. After all, anything that sits on multiple computers simultaneously is, by definition, backed up.

Well, it's a backup system, but the author's reasoning is wrong here, right? If you make a mistaken change to a file (like deleting it) and don't notice it before Dropbox is able to sync with your other computers, then all the copies are changed. The real backup lies on Dropbox's servers, where they store previous versions, not your computer.