As school children, one of the reasons why we loved Rand so much was because she was written so hilariously badly. The line about her legs was one of our favourites. She could have been great comic relief going forward if they had kept her as a day player.The disease encountered by the crew on Miri's planet was also able to affect their mentality! With both McCoy and Kirk losing their tempers on numerous occasions so why shouldn't Janice be complaining about her wanting Kirk to look at her legs only not with the repulsive purple blotches!!!
JB
Her role in Miri is largely decorative, similar in ways to the yeoman role in Galileo 7, Taste of Armageddon, or Operation Annihilate, but the fact that it's Rand elevates the role to something better. I suspect this is why people have affection for the supporting cast. We get used to the idiosyncrasies of the regulars and we enjoy that extra sass or extra humour that they bring.
Some guest characters also break through: Helen Noel and Riley are good examples (Riley's second appearance would have been a different character if Bruce Hyde hadn't been cast).