If things start going south in a public investment, a shareholder may not be able to exit that, either. The truth is, it's all about liquidity, and liquidity dries-up on the way down.
My original reply was intended to point out that what was the top article comment at the time completely missed the point. Cuban is arguing that severe liquidity restraints are bad, especially so for small time investors. For scenarios like you describe, public exchanges aren't perfect either, but they are very, very good at facilitating near-instantaneous liquidity and they strictly dominate the current set of private crowd funding vehicles.