i had australian passport and NICOP but they asked for pakistani passport, which i had with me but was expired and at singapore airport they used my expired passport with australian passport and were questioning me about NICOP
its in pakistan, about 1 hours drive from lahore.
I have used NICOP with my U.S passport when going to Pakistan recently without issues. I wasn't asked for it in the U.S or in Pakistan. Surprisingly enough, the airline officials at the ticket counters in the U.S were somewhat familiar with NICOP.
Oh and the back of the NICOP actually reads "Visa Free entry into Pakistan" or something like that. So technically, you shouldn't need the Paki passport if you are a foreign national with a foreign passport. All they should need is a "Visa" which is what the NICOP replaces.
I don't intend to renew my Pakistani passport any more once it expires.