As long as you can use THHN at the 75c ampacity and not a cable makes it clear efficiency and voltage drop is not the issue. NFPA seems unclear what the issue really is. The answer in the ROP was along the lines of "that is the way we do it and you have not convinced us to change".
I was really hoping for a real answer about what the difference was. I might have accepted "NM has a plastic jacket that doesn't contain the heat as well" but that wouldn't explain the rule for MC and AC.
