My biggest problem with "This Side of Paradise" is the issue of consent (something that's actual rampant in all Trek, TBH). Spock doesn't consent to being drugged. Leila being "high" doesn't excuse what she did to Spock.
If only that were discussed it the episode...
Does it need to be? The whole point is that what happened to the colonists was wrong, and that forcing the crew to submit to the same infection was just as wrong. See any other story where people are taken over by a mind-controlling force that compels them to infect others, like a zombie movie or, heck, a Borg episode. The fact that it's morally wrong is implicit.
A deleted scene from the script, which is in the James Blish adaptation, revealed that the spores were a kind of semi-sentient parasite mind-controlling their hosts, rather than just a "drug" or infection. So it definitely did address the issue of consent, in that the spores took people over without their consent and basically enslaved them with happiness. That's what Kirk was rebelling against. But since that dialogue was cut, people often perceive the episode as being just about some morally neutral psychoactive effect, or "drug" as proposed here. (I was familiar with the Blish version from childhood, so I never even registered that the key dialogue wasn't in the final episode. I was surprised to learn just a few years ago that many people perceive the spores as benign or morally neutral because they don't know they were meant to be sentient.)