Nothing against people doing things their own way. I could easily see a sculptor (Artist), writer (Journalist), and athlete (Performer) being their own careers instead of specialties under the Entertainer career. Then again, I can see a actor and athlete, both entertainer within the performer specialty, as separate careers.
The format of the careers does not lay them out that way though. What reason would there be for them to be called specializations/assignments if they are treated in every way like a career?
Personally having a military background, I even find it odd that one might be a assigned to Engineering for one tour and then to Sensors then Gunner then Pilot (service skills) as a Navy Engineering/Gunnery specialist. Why would the Navy of the future want to train people to have lower level skills in multiple things instead of being highly skilled in one thing? From my experience, it is uncommon for one to change their MOS.
It is not a simulation.
It is a game mechanic. A way to help create characters that are more rounded out in skills and not a one trick pony. Having somewhat related specializations that one can move between within a career helps with this game concept.
I don't require mustering out and requalification to change specialties. For me, I feel that the Character Generation Checklist makes it clear. It could easily have you go back to step 6 instead of step 5 if one was not supposed to be able to change specializations at the beginning of a new term in the same career.
Of course, the book is filled with errors and inconsistencies. On this very topic, the rules also state
In each career, there are three specialisations. Pick one of those when you enter the career.
The Careers summery table on page 9 lists the 3 specializations for each career. But take a look at every single career and they have 3
Assignments, not specialties. Page 8 talks about the tables and refers to
subtables corresponding to assignments
but here, if you go back to each career, the tables are not labeled as assignment but as specialist. shrug
That said, I'm not aware of anything else in the rules, that supports or opposes the one clear place that does address this issue, the character generation checklist.
But again, nothing wrong with people doing things the way that works best for them and their play group.