It's true that there are potentially dangerous incentives, and it won't work for startups with significant trade secrets. However, there are a large range of startups, and some people are even willing to share their idea in public! This would be sort of a middle-way. If you have a business idea that you'd prefer to not post publicly on Hacker News but you're not afraid of it being stolen by experience entrepreneurs, then this would be the way to go.

Lastly, although VCs are a good source of feedback, usually going and pitching your idea means a significant commitment (quit your job already, developing the idea). Furthermore, this form of feedback doesn't work for bootstrapped startups and startups in small markets.

May be he is afraid of the Mark Zuckerbergs

The issue is more along the lines of other startups attacking the same problem with more funding, particularly those still in stealth mode.