There were quite a few episodes where Rand could have contributed to the story by adding character to filler dialogue but apart from that, why replace her at the helm with an extra in the very next scene in the Naked Time?
You make some good points. The thing with the helm was, this was still 1966, and the Star Trek mindset was that you simply wouldn't show a woman driving when men were available but sitting back as passengers. She takes the wheel in a moment of emergency, but a man had to take over ASAP.
Women drove all over the place in real life, but the first season hadn't caught up. It wasn't the picture the producers wanted to paint.
We see a woman helmsman in "That Which Survives" I guess because American culture was changing so much, and Star Trek was trying to acknowledge it a bit. But not to overdo it. The show was behind the times on Women's Lib. For instance, there was no way they'd show a woman piloting the shuttlecraft while Kirk sat passively in a back seat.