The Prime Directive makes a lot of sense: let each planet develop its own culture. Once it is ready to spread among the stars, make friendly contact.
But the episode of TNG that I was referencing took it to a stupid, absurd degree. We need to protect the planet's culture, so we are going to let every person on the planet die! How is that "protecting" the culture? The culture will be extinct, along with all the people who developed it.
The ending of the episode was IMHO a cop-out. "Hey, the entire population of this world is one tribe of like 20 people. We can fit them all in a single holodeck, and they will never know they left their homeworld!" It would have been much more interesting, and more realistic, if they had tried to save as many people as they could, while minimizing cultural contamination (but unable to completely prevent it). So they would bring a few hundred to huddle in crowded spaces. (IIRC it's canon that you can fit hundreds of people inside the warp nacelles; the entire crew of the Enterprise NX-01 hid inside the nacelles in one episode of Enterprise.) "No, we are not the gods. The gods sent us to help you. No, don't worship us.... in fact forget us, and remember the hundreds of millions of your people we couldn't save. We're sorry we couldn't do more, but we aren't gods."
In the real world, sometimes all you can do is the best you can do and it isn't perfect. To me saving hundreds of people with some inevitable cultural contamination is a much more interesting story than saving 20 or so people without them ever realizing they left their home planet. And to me, the fact that they even considered the option of letting all the people die is stupid.