While still relatively rare in the interview process, they have become more popular, especially, people tell us, in Silicon Valley. It's possible that the prospective employer wants to discuss something that may turn out to be a trade secret and wants assurance that you will not discuss it with potential competitors or take it back to your own company. Of course, some still don't like to sign on the dotted line. Johnson doesn't sign them and doesn't want to work with companies who won't trust him. When he has refused, he says, the interviewer has backed down.
Companies fail because their execution stinks.
If you're confused by the scope of the agreement, ask the interviewer. Whatever your thoughts on NDAs, the agreements are usually legal when they don't require you to keep mum about everything . NDAs are usually enforceable when they only require you to not to discuss specific confidential information that you might not otherwise find out about the company's business or innovations. NDAs cannot legally require you to keep silent about illegal practices or information that could harm the public, and an NDA that requires you to be silent about everything would be too broad and could be challenged in court. When in doubt about an NDA, don't sign.