Why would you sign an NDA for a job interview?
Because they're Google. I know that's tautological, but I remember a blog post from some time ago about a guy complaining that all his friends that had been hired by Google had suddenly stopped chit-chatting about technical issues that they encountered at their workplace. They stopped doing that during the usual "at-a-drink-complaining-about-our-daily-job-ordeals" sessions, or on their blog, or anywhere else that might be publicly shared (and, why not? , maybe help other people learn from their (Google's) mistakes). The guy even compared Google with a "vortex", if I remember correctly.
HN discussion at the time: http://news.ycombinator.com/item?id=2362207
Don't do it. It's optional, you have the right to refuse. By signing you are putting a huge amount of liability on your shoulders for no reason. You're giving that company the right to sue you for practically any reason they can conjure up.
Them: We're going to tell you secret stuff during the interview. Promise not to blab? Me: Yeah, sure.
As far as non-competes and IP-assignment, from what I've seen these have gotten much more reasonable over the past 15 years. If you write or invent it on the job, it's a "work for hire" and the company owns it. What's wrong with that?
If you don't want to sign, that's fine. But there are probably 400 to 1000 other people interested in that open position, and 99% of them are going to sign the NDA.
Them: Please sign this NDA. You: No, I don't sign NDAs. Them (looking at the next guy in line): NEXT!
Yay!
It wasn't even offered to me - and this was for an engineering job. So I wouldn't say it's 'standard', even at Google. That said, I've also never seen anyone blogging about their Google interviews who hasn't signed one, so maybe they just forgot the paperwork for me!