[FWIW, a lot of the '60s drugs' weren't illegal for a long time. Acid started out being used for psychiatric purposes; Cary Grant took a lot of acid trips for that reason. John Wayne smoked pot, out on his yacht in the 1950s. There was not the stigma about taking drugs that there is now. And, compared to other stuff filmed in that era, this isn't really that bad. Really.]

A little context: in 1968, Star Trek TOS had been on for two years, it was iffy whether it would return, and the actors were looking around and doing fun things. Nimoy was, what, 30 or so? He was not an elder statesman; he was this obscure guy who for some reason looked good in pointy ears and green face paint. Why not do something silly and fun? Silly and fun were what was happening -- because the other side of what was happening was the Vietnam War.