Because it creates the worst brush combinations possible. Always build doorways yourself by copying and resizing brushes, never carve. As i sometimes have to disagree with Clavus, now i agree 100% with him! Even if you carve with a window or a door, you may accidentally carve other brushes,too. Also, if you carve a window, the brushes will be like this : otherwise it looks like this : .... i think so =D.