On the other hand, I don't want to immediately blame Dropbox just yet either. If you backup garbage (say, because you have disk corruption), then you can't blame Dropbox for backing up exactly what you told it to.
And Dropbox does offer a premium Packrat service if you want file history indefinitely. Perhaps the user can be blamed for assuming that he/she would only need 30 days of history, but this is really contingent on who caused the corruption to happen in the first place -- and that's unknown at the moment. [1]
A person running Time Machine (or similar incremental backup system) is a lot safer from this sort of problem than a free tier Dropbox user. Free Dropbox is better than nothing, but people can't keep assuming their files are safe because "they're in the cloud" and get synced to a few places.
Unless, as happened here, Dropbox erased the file and synced a blank version across all your computers. They they're safe and sound on any computer linked to your dropbox connection that hasn't been connected to the internet since the file got corrupted.
If you catch it in the 30 day window when they keep old versions you're find, but there are files in my Dropbox that I don't use every 30 days.