Only problem is, what you are suggesting, already exists in your favorite content creation tool such as Mudbox, Z-Brush, 3ds , Maya and others, is where you create your meshes, so asking the engine to add the features already present in these tools, would be redundant.
A BSP box will never be left as is in a game (No game of mine anyway), it will be converted into a static mesh, and then exported back into the creation tool set of your choice to be fixed up into something fancier.
Also the problem with cube2 is that it is still a cube, subdivided into smaller cubes. There is a real push these days to make more believable worlds, which in turn requires geometric shapes of all kinds. If you are making a shooter like doom, then straight walls, floor and ceiling could easily be created with BPS, only converted to static meshes, instead of replacing with custom meshes (I’m trying to say that more detail is worth adding, especially games that want to look realistic).
For example, you would use a landscape for your level instead of one huge BSP box, because the static mesh allows you to deform it in any way, trying to do that with a BSP is not really the right way to go about .
The only BSP’s I ever have in my level, are there to be placeholders, once I have a model to import, I delete the BSP, and move on to the next BSP to be removed.
Static meshes are what you need for creating a level, and landscape will take care of the terrain for you (unless is a game only in door, which isn’t as common these days) and is simple to use.
Just my 2 pennies!