I don't know why other crew members lower in rank to Spock, call him Mr.Spock instead of yes sir or Commander, either. Now that I think of it, I always wondered why they did that? Cause after all he is second in command, well from TOS to The Motion Picture he is and later he is Captain. Anyways, I guess I never payed attetion to that kind of thing. I would like to know that question myself, so your question is as good as mine.