I've never had any complete data losses until now, but I've had this situation where the Dropbox cache was the only place to retrieve files twice and it showed me that it can't be trusted this way. I'm certainly not gonna wait until the real deal happens just to have a personal story on how Dropbox can go horribly wrong. I've been shown the possibility and that's enough for me.

As for specifics, the case I mentioned is the one I saw where it can go wrong. IMO the only really save way to use Dropbox without additional backups is if you never share folders and only ever use one system with write-access at the same time - which is not the usual use-case of that product.