Red Ranger wrote:
I have another possible reason why Spock was most often called "Mister," or just "sir": That's the way he preferred it!
I believe earlier both Mr. T and the movie "They call me mister tibbs" were brought up. Both are examples of individuals who insist on being call 'Mister' as a show of respect. Gene Roddenberry's future is far from perfect, Spock is a vulcan in what appears to be a service dominated by (white) humans. There is sexism in TOS, and maybe still some specie-ism too.
In addition to being an expression of his non-militaristic pacifist believes, mister is a demand for a open show of respect.