It obviously is, but you're only trusting the 'cloud' to keep an extra copy and help you maintain synchronization between your copies. If they go down or out, you've only (potentially) lost the latest changes if you actually threw away the machine you were sitting in front of when you made them. In other words, good use of the cloud.

*edit: So they provide sharing and cloud sync, too. I'm not sure if that should detract. I haven't used Dropbox shared folders since they were Public Folders.