Here is my take on them. When I was searching boxes, the kids and I would try to fill up the album to make a complete set because otherwise they would get bored searching and not finding silver.
In doing those searches, we used to pull out any proofs or NIFCs that we found. However, eventually we just dumped them all back. I guess its possible that you could find someone who would be interested in buying them, an impaired proof on a common Kennedy or a NIFC that isn't mint state - is worth 50c. and not even worth keeping.
There are hoards of all of those coins so they aren't worth holding onto. It is interesting that people would pay a premium for NIFC coins from the mint and then spend them, but it could be that they didn't pass muster or they were dumped by someone who didn't buy them.