We've actually hit this before. "Out of Character", Nichols was not a regular cast member, but a 'day player' - a few steps up from an extra, but not by all that much. This limited the amount of time she could have on screen, due to union rules (which were and are very strict about such things).
That is a key bit of business that has been overlooked over the years, especially by Nichols, Takei, Doohan....not so much by Koenig, he's a big boy and knows the rules, but the others, in all the adulation that was rained upon them at the conventions, kinda forgot that there were only three regular cast members, Shatner, Nimoy, and Kelly, and Shatner was signed, and paid, as the star of the show.
The only other cast member who was signed as a regular was Grace Lee Whitney (note that she was in most of those first thirteen episodes, even if she just walked into a room and looked at somebody, she was there and got paid...until some unnamed lowlife studio exec tried to rape her and then had her fired, but that's another topic).
Hell, Eddie Paskey (Lt. Leslie) was in more episodes than George Takei, but I have yet to hear Paskey come out with a tell-all book about how Shatner is a rat bastard.