the PD developed out of the Vietnam War, and as a plot device to make resolutions of plots much more difficult.(you've got a landing party on a planet that hasn't developed phasers, you can't just beam down a security team armed with phasers, rescue the team, and go on your merry way)
Next Gen and modern Trek bastardized the PD beyond recognition to mean a ludicrously neo-isolationist policy of non-interference even in cases where common sense would favor interfering.
The world view has changed somewhat since the 60s...in the 70s there was a big movement in reaction to the "cowboy" mentality of the US for much of previous 200 years...I think there was a realization that the unhindered imperial colonization of the past was detrimental to the cultures that existed. Not much can be done about it now, but it makes sense to apply those lessons to future colonization of space. Unfortunately, TOS was a product of its time, and it still had the tendency to want police the galaxy, something the Vietnam war and other difficult to resolve "brush wars", unconventional, terrorist, ideaological wars have shown to be very difficult to solve conventionally.
Problem is..whose common sense? Common sense for us is not someone else's. There may well be totally alien world views out there, and what good is it applying our values to them? I'm afraid the idea of stopping at every world and deciding what's good FOR them is an outmoded idea already.
RAMA