This has to be viewed through the lens of the times in which is was produced for TOS. In that instance, it did it very well, from "Let that Be your Last Battlefield" to "Errand of Mercy" there are always strands of a different way for the audience to look at things.
The "Outcast" is a different series and is acknowledged by most to be bad. Not sure what that added to the discussion.
As @jaime asked, what would an R rating add?
And I feel that today, without the R rating Trek doesn't have much to offer. Other than at best tame social issues parallels or at worst inoffensive rehash and franchise history cannibalisation.
I would strongly disagreed that The Outcast is generally acknowledged as not bad. A large section of people have huge issues with that episode.