My two cents: I think NFC could help in terms of being a source of data, but I don’t feel that it should be the defining element. There are a variety of reasons for this, some of which have already been mentioned.
I seen it used in areas quite blindly and I feel that it makes the maps more confusing visually. I’ve seen major city roads that are neither state or US hwys that have lower speed limits be marked as “major highway” very close to limited-access roads with at-grade crossings and much higher speed limits also marked as “major highway”. To me, those roads are serving two entirely different purposes though they are classified the same. That makes no sense to me.
I also don’t see an issue with highways being classified as “minor highway” if they’re 55 mph with at-grade crossings even if they’re the only main route around. The point of minor/major/interstate visually is for drivers to see what they should expect when driving. If you make all US routes, all other NFC principal arterials, and current major highways classified as “major highway”, that seems like overkill to me. Because of this, saying “it’s the main road so it should be a major highway” bothers me. Besides, isn’t routing mostly based on average and real-time speeds? Maybe those newer, better routes are because of more users using Waze and providing more reliable, consistent data…
I also have a hard time believing that editors are simply making decisions on classifying roads only based on how they “look”. If they are, they are bad editors, someone should have a chat with them, and have them (re)read the wiki. These people are likely to not understand/have knowledge of NFC and would mess things up anyways.
I do think that we should totally make better use of the “primary road” distinction. Though, for me I’ve been trying to clean up areas first, and then go back to add primary roads as these didn’t really import well from the beginning and is probably the most under-utilized road type (in my opinion).
As I said, these are my two cents. Feel free to discuss as you wish.
My final thoughts - When Michigan figures out how to build decent roads and make left turns like the rest of the world, then we MIGHT listen. :lol: 
PS - it’s getting late for me - if I seem incoherent, let me know!