I think this bug should be considered completely separately from how unwise it is to use a cloud service as the sole storage of important files. Regardless of the circumstances, losing user files against the wishes of the user is the absolute worst thing a cloud backup provider can do . Even for files that are deleted intentionally and unambiguously by the user, I'm astonished that Dropbox actually deletes the files…
I don't think that they are. More like the modern incarnation of a file server. Like a fileserver, they have resiliency against service failure or individual storage system failure (via S3). Cool stuff, but it ain't backup.