You know what though, when you start adding grey areas then you start muddying the waters. Then you increase the chance that real damage may occur.
If total avoidance were the only way to deal with risk, we would've given up using fire and would still be huddling in the cold and getting sick from uncooked food. We would've given up flying after the first plane crash. Moderation is not wrong. Recklessness and total avoidance are the extremes, and like all extremes, they're both unhealthy in their own ways. There's always a smarter middle ground, one where you avoid damage by learning how to
manage a risk rather than just avoiding it. When people and societies grow up, they're supposed to learn how to do things better, to do them right and responsibly, rather than just giving up on even trying.
And absolutism itself does damage, by being too easy and sparing you from having to think or make decisions, thereby leading to overly simplistic and inflexible responses to complex situations. If you just refuse to consider the options, you may end up unthinkingly taking a path that does more harm than good, as in "Pen Pals" and "Homeward." It's the opposite of being responsible.
I do think leaving a society be is the best route to take. The Federation can still observe and learn from those societies, but it's best I think to avoid direct contact whenever possible until they are more advanced.
And I still say that's based on a lot of false assumptions about how cultural contact works, derived from the way
Star Trek tells stories designed to sell the Prime Directive, which is circular reasoning. When you study actual world history, as I have, you see how bogus and simplistic a lot of those assumptions are. Interaction between different cultures is normal and healthy. It's not some "contamination" that inevitably leads to harm -- on the contrary, the societies that are most exposed to outside ideas are the most robust and dynamic.
It's no different from interaction between individuals. We're better off connecting with other people than going it alone -- as long as everyone respects everyone's boundaries and freedom of choice. You can avoid meddling in someone's life without hiding your very existence from them. You just have to avoid thinking you're entitled to impose your beliefs on them. Again, the PD isn't supposed to be about other cultures' immaturity, it's about
our immaturity in the face of the temptation to play god. But one hopes our society would eventually mature enough that we could resist that temptation without having to avoid it altogether. The PD should just be the first, simplest way of managing that temptation, not the final word on the subject for all eternity.
And we know Starfleet has engaged in passive observations, or even occasionally direct observation through the use of observers. I liked Greg Cox's "The Antares Maelstrom". He had observers living on the planet and observing them, but being very careful to basically stay out of the societies way. Going so far as to live as the natives do. Now, that takes a special kind of scientist, one that is willing to basically live another life, and one that is able to exercise restraint. It'd be easy to 'want' to interfere in some ways. But it's important to note what we may think is a mistake may turn out not to be at the end of the day.
But that's got the same problem -- unilaterally acting without the consent of the natives. That is not protecting their freedom, it's violating it. It's also utterly the wrong way to do immersion anthropology. You can't do it if you hide your true nature from the people you embed yourself with. They have to know that you're an outsider so that they can show you how they live, and you have to be honest and accepting if you want them to trust you enough to show you. Moreover, the last thing you have to worry about is interfering with them or changing their way of life. They're in their own home and they outnumber you. They control the interaction, and the context reinforces their worldview and customs over yours. They're the ones who will change you, not the other way around.
That's literally the goal of immersion anthropology, to learn how to think like the people you're studying, to become like them rather than making them like you. And you can't do that if you hide from them, if you put on an act that's your own unilaterally constructed version of what you think they're like. It's condescending to them to think that could even succeed at fooling them. No, the only way it works is if you come to them honestly as an outsider and let them reshape you.
This is what "Friday's Child" got right and so many other episodes (especially "A Piece of the Action") got wrong. Cultures don't abandon their existing beliefs and values easily, not unless they already have an internal incentive to do so. Just meeting them and trading with them doesn't disrupt anything. The Capellans didn't change a thing about their way of life in response to being contacted by offworld miners and Starfleet medical teams. They still acted exactly as before, expected outsiders to conform to their customs and rules, and showed little interest in the foreign ideas the Federation offered. Even when Kras interfered by setting a coup in motion, he just nudged an internal political conflict that already existed, and as soon as Maab got what he wanted, he tossed Kras overboard and just did his own thing. And in the end, despite the interference by Kirk's party and Kras, it was the Capellans who resolved their own crisis -- Maab corrected his mistake by sacrificing himself so Kras would be killed by Maab's lieutenant. And their culture went on the same as before, even though their new teer was named for offworlders.
For all that "Friday's Child" was steeped in '60s cliches of tribal "noble warrior" cultures, it's actually one of the more plausible pieces of anthropology in the Trek franchise, and a sterling illustration of why even the active attempt to interfere in another culture won't automatically change it, because cultures have inertia and resistance to unwanted change. Or rather, when they do change, it's because of internal factors that create an incentive to change, and any external influence will only be embraced if it serves an existing internal drive for change (e.g. Maab accepting Kras's support for the coup he already planned, or the Mexica accepting Cortez's assistance in overthrowing the Aztec rulers they loathed).