In a lot of cases Bethesda decided to make meshes with combined blocks that have no simple way to uncombine without Blender. Case in point the CaveGMid piece with a hole. Why they made it combined with the floor instead of separate I have no idea esp as there IS a separate floor piece if you wanted one there. You can remove part of the floor but when you remove the scattered rocks on the floor the ceiling goes also. This is really stupid. And not needed. Anyway I removed both pieces and will fill the ceiling gap with boulders. But really. Why do they do this? Anyway, just venting.

i think the reason is that, when they make stuff with the same texture one block, it'll have to load that texture once. if they make a seperate block for every piece, the texture will have to load once for each of these, hence consuming more memory.

but i'm far from being a modeling expert anyway, i'm sure people who actually know stuff about models can tell you more

That's not quite right on the technical level, but it's the general gist: meticulous optimization of resource usage in assets. A separate NiTriShape would make it bigger, as that new NiTriShape would need separate block headers and everything. On one mesh it seems like nothing, but with all the other assets subject to this as well... it can make a pretty big impact. These assets have to be usable on consoles, not just PCs. Assets in realtime animation engines in general need to be more optimized than still frames or non-realtime animated renderings, since it has to do it in...yaknow... realtime. It doesn't have all the time it needs to render, it needs to get 20, 30, 40, 60+ frames up every second.

Still it's frustrating. You want to make a hole over water and there is that dratted floor piece floating in the middle of your pond. I was not suggesting a separate block in the nif but a ceiling hole nif with NO floor block at all including the scattered rocks on the ground. At least as an alternative option since there is a separate floor piece you could place if you wanted one there. It would have to use less resources than what they often did by simulating it with an empty section filled with a dozen boulders to make a hole in a ceiling.