I think it matters who tells you it's a stupid idea. If it's investors, you can safely ignore them; they aren't looking for what you're selling anyway. If it's potential customers, however, you probably have a problem.

In AirBnB's case, they had already validated the market. They had their first 3 customers before they even thought it could be a business. If people are willing to pay you money for your idea, it really doesn't matter what investors think.