I will note that Dropbox has changed so much from when I first learned of their service. They seem very focused on providing solutions to businesses now compared to back then when it seemed more like a product for individual users.
Because the margin on enterprises is so much higher. Most enterprises will pay money to make a problem go away whereas consumers will look for alternatives.
I'm willing to bet they make $100K+ every round they send these out. Everyone has to consider at least what to do with the email (ignore? accept if it's useful again? maybe just to stop getting bugged)