One parallel Earth story was marginally okay, but several scripts of the same genre was just too much to swallow. NBC pressured GR to show the viewers strange new worlds and civilizations.
There were conflicting pressures -- on the one hand, to show new things, but on the other hand, to save money. Remember, the parallel Earths were not an afterthought. The concept was integral to Roddenberry's initial series pitch. It was part of what sold the show in the first place, because it made the show affordable. It's spelled out on page 4 of the original 1964 pitch document:
The "Parallel Worlds" concept makes production practical by permitting action-adventure science fiction at a practical budget figure via the use of available "earth" castine [sic], sets, locations, costuming, and so on.
AS important (and perhaps even more so in many ways) the "Parallel Worlds" concept tends to keep even the most imaginative stories within the general audiences famee [sic] of reference through such recognizable and identifiable casting, sets, and costuming.
This is what NBC bought up front. It was always meant to be a core element of the show. That initial format document includes summaries of several possible parallel-Earth episodes, including "President Capone" (basis for "A Piece of the Action"); a caveman/dinosaur episode; a world paralleling 1964 but with an Orwellian police state in charge; a world whose inhabitants had duplicated important figures from Earth history as gladiators (perhaps an antecedent of "Shore Leave"); the self-explanatory "Camelot Revisited"; a "frontier log-fort" colony; a planet "exactly duplicating St. Louis, 1910" except with women enslaving men; and a planet duplicating the plantation-era South but with blacks enslaving whites.
Seeing a re-hash of National Socialist Germany or Ancient Rome was pretty much schlock television.
To the modern eye, yes. To viewers in the 1960s, not so much. It wasn't so different from what you might find in a
Twilight Zone episode or a B movie of the previous decade. Even the prose science fiction from earlier decades (and ST drew heavily on the pulps of the '30s-'40s) often portrayed alien worlds as skewed variants on Earth cultures or environments. It was more about using the exotic settings as allegories on aspects of human nature and society, in the tradition of Swift's
Gulliver's Travels. (And
Gulliver's Travels was actually a working title for
Star Trek. Swift's work was very influential on Roddenberry and Herb Solow as they developed the show. The use of captain's logs derived from the idea of taking a "traveler's tales" approach.)
And the explanation behind the parallel in "Patterns of Force" made more sense than a lot of them. This wasn't a planet that just coincidentally duplicated an Earth culture the way the "Bread and Circuses" Roman planet did. This was a planet that had fallen under the influence of a misguided human who deliberately recreated Nazi Germany due to a gross misunderstanding of its merits and dangers, and who thereby resurrected the horror that lay at the foundation of that state. It's a chilling concept, and not completely implausible, given how many Neo-Nazis and Holocaust deniers are out there today.