One thing that has to be taken into account, and that has been mentioned earlier, is that the restricted space of a barracks is suitable for military ground pounders and needs to come with expansive other spaces for them to spread out in. The barracks are simply rack space, not living space.
Barracks are suitable as temporary berthing space. A marine assigned to a ship for a 6 month deployment of said ship would be considered a passenger, albeit a longer-term one. I have no issue with them being assigned barracks. If a Marine is assigned as crew (not temporary) then they should be treated exactly like the naval crew of similar rank.
Tell that to mongoose. Yes troops assigned to a ship should have the same cubeage as sailors and transported troops barracks just like in the real navy just like on a submarine. Common sense says they should be revised so all E-4 and below are housed in Barracks just like real life.
I don't think we are on the same wavelength for the discussion here. I posted up-thread an example of crew quarters aboard the BB-Texas - it was very similar to the pic above.
I've tried to explain where the difference is, and where the rules are not well written to reflect what should be normalcy. I will try one more time, and if this doesn't work I don't see there being any more utility in discussing further as we don't seem to be communicating our points well.
To explain it better I'm going to set up a scenario - We have the good ship Lollipop, a planetary assault ship. There are three individuals, all of the same rank - Bob, Mary and Larry.
Bob is assigned as a cook. He cooks every watch and has damage control duties during combat.
Mary is assigned as a shipboard Marine for the Lollipop. Her duties can include shipboard security, security in port, boarding actions, or damage control party assignments during combat.
Larry is also a Marine, but he's being transported by the Lollipop to invade some dastardly enemy of the Imperium.
Bob has a cabin and has regular duties/watches aboard the ship. He is full-time naval crew.
Mary has a cabin and has regular duties/watches aboard the ship. She is full-time Marine crew.
Larry is in a barracks space with the other Marines being transported on the ship. Upon arrival at their destination he'll leave the ship.
Bob and Mary, being crew and equal rank, should have equal quarters aboard the ship. Larry, not being crew, only needs the barracks because he's essentially living cargo.
The duties of Bob and Mary are different, however both are assigned to the ship and go where the ship goes and will remain as crew until they are transferred to a new duty station. Larry isn't assigned to the ship and leaves when the ship reaches its destination. These are two entirely different crewing scenarios - assigned to the ship vs being transported by the ship.
Under the rules as written, Larry and Mary would both get barracks as Marines, but Bob gets a cabin. And this is where I take issue with the rules. Crew is crew, passengers are passengers. Treating Larry and Mary the same is illogical - one is assigned to the ship and one is not. Treating Bob and Mary the same is logical - both are assigned as crew and neither is a passenger.
The above explanation holds true today for submariner crews, and used to hold true for ships that actually embarked Marines permanently as regular crew - they received equal treatment as far as cubeage goes based on their rank - not their job function.
Crew is crew, and passengers are passengers. Doesn't matter what their job functions are, if they are assigned to the ship then all crew of similar rank should be treated the same regardless of their job functions aboard the ship.
Travel times, for crew at least, are irrelevant insofar as a ship having the necessary capabilities to support and entertain its crews for long periods between ports. I referred to a year in reply to your statement about troops being transported from Core to the Marches.