The question you're asking is, "why does it make sense for YC companies to be in stealth mode?" Sometimes it makes sense to be stealthy, and occasionally you need to hire folks when you're in that state. It's certainly a handicap (as spolsky says elsewhere in this thread, what kind of hotshot would want to interview without knowing ANYTHING about a company?).

Good post on benefits of stealth mode here: http://trada.com/blog/2010/03/31/the-stealth-mode-trada%E2%8...