It just popped up in my mind.
- You could decorate any room as you like. You don’t even need to step out of it most of the times.
- Other people can be projected inside it like Voyager’s doctor.
- Also rooms could be much smaller. They only need to be big enough a human(oid) can fit inside.
- In emergency cases most holograms can be shut off to match increased energy demands by weapons and shields. You only really need seating/bed and a (non-exploding) console screen.
- Much of the specialized rooms like a bar, med bay, etc. won’t be needed anymore as a holodeck can imitate all of them.
It irritated me a bit that a Discovery gets fancy floating warp nacelles but holodecks are… wait, does Discovery has a holodeck? I don’t remember seeing one.
Holograms aren’t stable in the long term. They will start to come apart after some time. That, and they constantly require power to maintain. A bed and furniture does not, and will still work if the ship needs to go without power for one reason or another. Most things someone might put by hologram can be done by replicating the thing, instead of using a hologram. Most rooms have a replicator, and excepting furniture, which you might need to ask engineering to make for you, you can just make it yourself.
Starships aren’t lacking space by any means, so there’s no need to stick people into a broom closet.
Though there are things like that. The Ba’ul “migration” ship was basically that, where the entire ship was meant to be a holodeck. In the 32nd century, rooms are basically holograms, except that holography has been superseded by programmable matter.