That's crazy. Unless they're hiring you to solve a novel problem there's really no reason. If you're not paying me for the interview, I'm not signing an NDA.
From the interviewing side -- if you're under NDA I feel more free to answer arbitrary questions about the business. But your opinion is not unique, so my current company stopped asking for NDAs. We sometimes still do it during the offer stage if a candidate has a lot of detailed business questions
Yeah, I could totally see it being valid if I start asking for business model or implementation details. The only time I thinknid ask something that detailed would be if it's a startup and I want to understand their financial situation to see if they're stable.