Like I said, it was a mess. And now nearly 50 years later some of us are trying to "unmess" it. Good luck with that!
Bob Justman said it best when talking about this episode... (paraphrasing) 'When we had a bad episode, it could almost always be traced back to a bad script.' I'll take it further myself, as it appears to moi that in this instance a bad script can be traced back to convoluted ideas.
I'll just mention it here rather than give it it's own 'lite on content thread'...
"Miri": They run across a planet that looks just like Earth....and it's never mentioned again. WTH was that all about?? It's set up in the teaser as being the plot of the episode and is never mentioned again. Bizarre.
(Goes to Memory Alpha) Ha! I see that Christopher Bennett came up with an explanation!