Okay, then...I understand where you're coming from, but it makes more sense to justify reasoning behind something that has reasoning behind its design, rather than try to justify reasoning for something that had no thought behind it besides 'make it look big.' It might even be admirable that you make the try, but face it, only TIMO can come up with any decent reasons for there to be doors on this dumb thing, and that still isn't good enough, because the doors are there just so there is an obstacle to escape. If the ship parked on the outside, you wouldn't have the 'warningspacedoorsareclosed' bit ...
Instead of thinking in terms of what we've seen on-screen in Star Trek at all... consider why you might want to build an enclosed volume of that sort in space at all. Think "real life" rather than "Star Trek."
What advantages would an enclosed volume of that sort give you? Would it give you any?
Here are a few I can think of.
1) Protection from micrometeorites and other debris
2) Protection from radiation (both EM and "hard" types)
3) The ability to recover lost toolbelts?
4) Provision of a working atmosphere.
Anything else?
And are there any of those which wouldn't be tremendously useful during construction and/or repair operations?
IGNORE the thought that we're talking "Star Trek" and think reality. If, in 250 years or so, we have something like this really in orbit... what purpose would it serve? Any answer you can come up with which makes logical sense can be tossed out... except "in order to tell a cool story."