You are suggesting that arbitrary geometry be split into a BSP tree or portals. This defeats the memory usage and speed advantages of referenced map meshes.
Referenced meshes allow you to have a scene consisting of 1,000,000 polygons, while actually only storing 50,000 polygons in memory. By locking a vertex array, and rendering all instances of that mesh, your rendering speed is limited only by fillrate.
Using unique geometry throughout a scene is like using unique textures on everything. It uses far more memory, it renders much slower, and it takes much longer to produce the media.
Referenced meshes allow you to have a scene consisting of 1,000,000 polygons, while actually only storing 50,000 polygons in memory. By locking a vertex array, and rendering all instances of that mesh, your rendering speed is limited only by fillrate.
Using unique geometry throughout a scene is like using unique textures on everything. It uses far more memory, it renders much slower, and it takes much longer to produce the media.