Without having NAICS codes in the core or AML system, I'm not sure I would agree with that one. I don't want to dig through CDD and compare NAICS codes for every customer. If you do have the NAICS code recorded already, then I agree that it's required regardless of whether the field is critical.
If you don't have a business' ID number on file, but it's publicly available on the SOS, would they expect you to go out and get that too? These are the kinds of things that make CTRs expensive for smaller banks.
I agree with your point about picking battles though, if that one's easy enough to implement, better to just do it and have better CTRs and no questions from your examiners.