I agree. Any decent professional should know when to keep his mouth shut. And it should know that leaking privileged information would potentially get him fire

I agree. Any decent professional should know when to keep his mouth shut. And it should know that leaking privileged information would potentially get him fired and marked as unreliable. No NDA required. An NDA not only keeps an employee quiet, but it makes him liable long after the employment relationship has ended. I, for one, would not work for any company that would force me to sign an NDA. Besides being insulting, it's just not worth it.