Not to mention that the ability to raise additional money after YC would be significantly impeded by being cagey about the ideas behind the company. Almost every investor I've ever heard speak on the subject has said they simply do not sign NDAs. If someone expects YC to sign an NDA, they would likely expect every other potential investor to sign an NDA...and that's not going to happen.
I suspect that investors not being willing to sign NDAs is at least as much attributable to the fact that people who want random people to sign NDAs have too much confidence that their idea alone has value, and thus won't understand that execution is vital, and likely will not be willing to evolve their idea as the market demands. Well, that, and "idea" people are just generally useless and annoying. So, asking for an NDA is a sign that you're a useless non-starter who won't be able to build a company or compete in the market (because your idea has to be public before you can do anything with it; if you're concerned that your only advantage is secrecy, then it seems likely to me that failure is imminent).
I suspect that investors not being willing to sign NDAs is at least as much attributable to the fact that people who want random people to sign NDAs have too much confidence that their idea alone has value, and thus won't understand that execution is vital, and likely will not be willing to evolve their idea as the market demands. Well, that, and "idea" people are just generally useless and annoying. So, asking for an NDA is a sign that you're a useless non-starter who won't be able to build a company or compete in the market (because your idea has to be public before you can do anything with it; if you're concerned that your only advantage is secrecy, then it seems likely to me that failure is imminent).