The only occasion I've ever found justified for hiding the company name is when you're hiring in a town where the unemployment rate is > 25% and you can't manage the unemployment mob that inevitably forms outside the store.
There is only one reason those anonymous job postings exist - lazy hiring managers. I really wish pg would put his foot down and tell people to start standing up for their brand rather than hiding from it.
If an organization expects me to send a detailed resume to them without knowing the first thing about them, that starts the entire relationship off in a profoundly asymmetrical way.
I.e. Techcrunchs only reason for existence is to suck their own dick.
Now, YC companies seem the least likely to have anything to hide about themselves. That said, when I see an ad like this, my first reaction is to shy away - because even if they do have some legit reason for staying stealth (competitiveness, hiding YC funding, etc), they could also be doing this just to hide some fact that I might find negative later in the interview process. I've been burned one too many times by non-YC companies doing this to me, that I'm concerned to see YC companies doing it too.