I suspect what that comment means is that Dropbox have traded security for ease of use at a fundamental level - compared to something like tarsnap (which is dedicated to doing backups in a very secure way) Dropbox does look a lot less secure but it is also a lot easier to use and supports a much wider ranges of uses.

Edit: I am a pretty happy DropBox user, but I only store files there that I wouldn't really care too much if they were made public. DropBox to me is about convenience, not about security.

Edit2: I was a pretty happy DropBox user, now that I think about what having a system that is prone to "failing open" means I'm seriously thinking about closing my account. However, I'll wait and see what the "official" explanation is first.