In my opinion a prototype is used to prove there is a market for your product and attract seed investment.

An MVP is then created with the help of that seed investment and launched to help greater substantiate your product and attract further funding.

The Dropbox video is a good idea. It may not be a prototype but it created enough hype for them to show investors that there was a market and demand for what they were building. My only concern is that if you are creating a product in a new market you are potentially giving away a lot of your first mover advantage with that tactic.