Eh, I heard somewhere that the first few times Shatner shaved, he complained because it itched so badly. And to get that perfectly smooth sheen, it had to have been a pretty intensive process.
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.