Except there would be no reason to take Rand down. Kirk needed someone that was an actual expert to tell him what would be wrong on a penal colony.
Using Rand would've made zero sense in this instance.
No, I think that's a poor excuse. Rand's exact qualifications were never elaborated upon, although based on her comments in Miri, she was at least trained in basic astronomy. Trek has a long history of using main characters when there is no logical reason for them to be there. In none of the shows featuring yeomen is there any reason for a secretary to be part of a landing party. Look at the Galileo 7. What was the reason for anyone other than Spock, an astrophysicist, a pilot, and an engineer on that mission? Sure, they could have included a couple of biologists if they were intending to conduct experiments but there was no evidence that they intended to do anything other than sit in their chairs. If Mears was a technician as well as a secretary (and this is the direction that they went later in the movies with Rand) then there would be more logic but she does zero to help Scotty. Spock is a physicist and computer expert but frequently joins landing parties in preference to a geologist, botanist, biologist, or anthropologist. Even the M5 acknowledged that Kirk should stay on the ship a lot more. If you take it to its logical conclusion, you'd need a lot more guest stars every week.
Similarly, why not use Noel or another staff psychiatrist in Whom Gods Destroy? Or in And the Children Shall Lead? The Alternative Factor? And probably at least half a dozen other episodes where McCoy just bumbles along?
There are also a few episodes where Chapel, a research biologist, spends her time handing McCoy slides to examine when she is probably better qualified to be taking the lead in the research.
Personally, I think there is a plausible reason to include Kirk's yeoman on any diplomatic mission. She could also have given exposition as the 'records officer' in a couple of episodes. If we assume that she is security trained she could even double as Kirk's personal security escort on more dangerous missions. The problem with the yeoman character was really the blinkered writing of the time rather than story logic.
Going back to this episode, the only real story reason Noel was there was so that Kirk could be manipulated into believing he loved her, and that would have worked for any other woman. Everything else was padding.
