It's a good point about Spock being rarely shirtless, though. While the makeup issue is probably correct, I think his constantly being clothed also adds to two important traits: his alien-ness/mystique of the unknown, and his discomfort in being himself. By keeping his body literally under wraps, he distances himself from others and shows that he is not at peace in his own skin. It fits perfectly.
That makes a lot of sense.
Are we sure they didn't let Nimoy take off his shirt because Shatner wanted to be the only shirtless one?
Well, there was Sulu that one time. (Or was it a couple times?)