I can't find any authoritative discussion of Spock's intended purpose. I asked someone who's seen the show in as non-loaded a way as I could, and ey said that Spock was generally intended to be perceived as rational, and that the chess games in particular are often a metaphor for the action of the episode. McCoy and Spock often function as Kirk's System 1 and System 2, giving him advice that he combines into an instrumentally rational decision. I agree that Spock is often a good example of what not to do.