This is in India. They sent me the NDA copy which states 5 years expiration :) Isn't this something to be done before joining the company and not post quitting? I told them that I have no intention of competing with them nor stealing their ideas/customers. But they are very insistent and almost pestering me now.

I certainly would not, as you would be more restricted in your freedom to communicate and opening yourself up to more possibilities of legal troubles down the road.

They would love it if you signed, but I can't see any benefit in it for you.

If they are pestering you, the NO, definitely not. Why would you sign something at this point? You already quit.

Cost/benefit? What's the cost, and what's the benefit?

From your description, it appears they are asking you to take on a "cost" with no corresponding benefit. Fail.

If/As you are under no legal obligation to do so, why do so? Is there some professional, relationship, or other benefit to doing so that you've not yet described here?

I am not a lawyer so take my advice with a massive grain of salt...

It would help if you post where you're located as laws varies by state and if you're not in the US, even more so. I don't believe you're under any obligation to and I'm sure they're asking to ensure you don't leak anything. If I were you, I'd just clarify with them but wouldn't sign.

You have no obligation to sign the NDA at this point. If you want to keep a positive relationship with this company and it has nothing to do with your new job then you might consider signing it. I would do so only after having a lawyer take a quick look, though. I would also make sure it has an expiration, like 1 year.

I can't consult you on this, but I do recommend you to speak with a lawyer. Ask around the developers you trust for recommendations.