I wasn't inferring that Nurse Chapel and Zarabeth were played by the same character! I just meant he USES those two women rather frequently in different roles!!! I may be a bit dim, but I'm not THAT stupid.
I think you're using the word "character" when you mean "actress." That's the source of our confusion.
Roddenberry did give Majel Barrett a role in every show he ever made. As for Mariette Hartley, though, your premise is false. As I said, Roddenberry had little involvement in the third season, so casting Hartley may not have been his decision. She was a popular actress at the time and showed up in many productions. And sorry,
Therin, but you're mistaken;
Earth II was not a Roddenberry production. You must be mixing it up with
Planet Earth, the retooled version of
Genesis II. Hartley only appeared in two Roddenberry-created shows,
Star Trek and
Genesis II, and Roddenberry may not have had any say in her casting in ST, because he'd stepped back from direct involvement in the show by that point. So there is no pattern of Roddenberry reusing Hartley.