Interesting. I think the captain's yeoman would have been a justifiable addition in any of the episodes featuring a trial or crew assessment (Court Martial, the Menagerie, the Deadly Years, Wolf in the Fold, and Turnabout Intruder); any episode where Kirk was specifically and publicly in peril (Arena, the Savage Curtain, Tholian Web, and the Enterprise Incident, to name a few); any episode where the whole crew is captive (I, Mudd, this Side of Paradise), and as part of any diplomatic landing party or diplomatic story (a Taste of Armageddon, Journey to Babel).
For Grace specifically, I would have also singled her out to appear in comedy episodes (I, Mudd, the Trouble with Tribbles, A Piece of the Action) because she had great comic timing.
For me personally, I would not have used them on exploration duty, I would have used Palamas, Mulhall, or a female security guard as appropriate. Using them in those fish out of water scenarios definitely made them look pointless and there to fulfil damsel in distress roles. Mears' only job was to switch on the tricorder, Zahra was the only unarmed crewman (even McCoy had a phaser), and Landon was the only crewman who wasn't an officer and was only there to teach the natives about snoo snoo because Chekov wasn't into beefy security guards.
I would also have used them as personal shuttle pilot for the captain although that might be covered under diplomatic missions above (Metamorphosis).
Far from being difficult to write for, I think there was a lot of story potential for the yeoman in quite a few different scenarios if they had made her more independent and stuck with someone plucky like Colt, who I can imagine disposing of the overloading phaser in Conscience of the King, or sussing Lester in Turnabout Intruder. The yeoman character suffered because they obsessed over her crush on Kirk to the detriment of everything else.