Oh but we do, in "The Tholian Web." Replacing those shelves with another drawers unit would provide storage for two person occupancy.
Thanks for refreshing my memory! But now that you mentioned the idea of the diplomatic cabin with a high possibility or probability of an adjacent bathroom (for Elaan to hide in) I do think Uhura’s cabin is, indeed, special and rather unique.
This second shelf compartment left of the drawer / mirror section looks rather empty as if Uhura had just recently moved in there to put a few of her items there - previously on the shelves of a standard cabin. Since her bathroom would probably have a toilet, there’s no need to have one inside Uhura’s rotating closet (which I still think would work / be applicable for most standard cabin wardrobe cylinders), but I’m certain that storage space came in handy, when all the gifts for and/or the luggage of Elaan of Troyius had to be stashed somewhere.
Given the obvious special nature of her cabin, I don’t think we can draw conclusions that apply for all the other and/or standard ones.
Which might raise a question on how to accommodate 100+ passengers in "Journey to Babel." Easy: You leave your exploratory specialists at the nearest Starbase.
That’s a good explanation. Or the crew moved temporarily into rooms with bunk beds, designed for colonist evacuation or detention of prisoners. I’d like to believe that the (unseen) back part of the “crew lounge” in “Day of the Dove” (see Deck 6 draft in post # 93) did contain an adjacent room with bunk beds to accommodate the 40 Klingon survivors.
So in fact it is the Briefing Room doors that some stagehand has left open, not the green middle door as I thought. There were quite a few of these GNDN doors on the corridor set, no doubt added to break up the expanse of grey. However, given that most are blocked by the presence of existing rooms, they come off as little more than very shallow cupboards! Incidentally, there is room for a thin cupboard and Kirk's TCM closet there.
While it is the set of the Briefing Room, I’d like to think that on several decks where we don’t see what’s behind doors it is also a crew accommodation room – which could help to rationalize the open door you spotted. Let’s imagine it opens to a narrow corridor for, let’s say 4 single cabins. As long as there is still one crew member in his cabin, the door stays open, so everybody will know who gets his behind kicked for being late to his or her work shift.

Bob