I was assuming dropbox would do reference counting so they could delete data that nobody is using, and account tracking so they can put storage limits on your account and charge you for using more space.

Definitely. There are a few problems that can "de-anonymize" users. I don't claim that the outline presented would be very robust.

Maybe. It depends what you are guarding against.

I agree, that requires more qualification. I think in the context of subpoenas or trawling, there are some definite advantages.