Umm, if they were near Ceti Alpha (or more properly, Alpha Ceti), then they were only about 250 light-years from Earth -- and given that the Botany Bay was a pre-warp vessel which had been in slower-than-light flight for about 270 years, it really couldn't have been much farther away than that anyway.
As you may have noticed, I know next to nothing about space science. I guess I should have used the ever-reliable ''bajillion'' instead of a random number like 6000...
The only time Kirk's Enterprise was any significant distance outside the Milky Way was in "By Any Other Name" courtesy of the Kelvans. Otherwise, pretty much the entirety of TOS took place within a very small portion of a single spiral arm of the Milky Way. It's a very big galaxy.
I bow before you, sir...though what about WHERE NO MAN HAS GONE BEFORE? Didn't they cross the great barrier and exit onto the neighbouring galaxy? But I digress...
Besides, why assume she was just an Earth historian? Kirk only said she was familiar with late 20th-century Earth; he never said it was her exclusive area of study.
I dunno, I'm pretty sure that in the 60's the writers (including Gene Coon if he was on staff at the time) probably DID mean it and never gave it an afterthought. All humour aside, if Star Trek were created today the members of a starship's crew would probably be envisioned as very similar to the Apollo missions flight crewmen (as elaborated in the excellent series FROM THE EARTH TO THE MOON): outstanding physical specimens with a very clear specialty (pilot, science, etc.), but each trained extensively in related sciences/tasks such as geology, zero gravity walking, first aid, emergency procedures, basic piloting skills, space science and physical science, etc.
I'm going to throw some numbers here, but say for a ship's complement of roughly 400, you have about 100 engineers that keep the ship running, 100 scientists who handle everything from routine analysis to research, 50 medical personnel to treat illnesses to biological analysis, 75 operations crew to handle what keeps the ship going, and 75 security personnel to guard high-risk areas, accompany to planetside and secure against incursions...
(please don't chime in to quibble over my numbers, they are totally random, people)
So with 75 security men, it's safe to assume that on most calm and quiet days only a small fraction of them would be assigned to a post of some kind. Logic dictates these crewmen can't sit on their asses all day, and they fill some sort of duty at another department, for example among the phaser crews, weapons details, etc.
So yes I agree, to finally get back to McGivers, being a historian was probably a minor part of her ship's function...
Besides, being a historian isn't just about reciting memorized facts. The actual job of a historian is to research historical texts, records, and other accounts and formulate models and interpretations of the past based on that evidence... Presumably the job of a starship historian would be to study the historical records and documents that exist on the alien planets visited by the ship, or to interview the natives and collect oral history, and then formulate theories about the planets' historical and social development
I can't disagree with anything you say, because I love history as a subject, and as reading matter. To properly analyse any new planet and civilization, a ship WOULD require the efforts of teams of geologists, biologists, botanists and historians to gather a complete picture.
That said, I based my comment towards McGivers on two things: the Captain's OWN dismissive remark on her, and her total lack of eagerness when summoned to a landing party (''hey McGivers! you finally get a cherry assignment!!'' ''(aww, I had better things to do...ummm)...on my way!''