Leave it to BMW to over-complicate their cars. But rather than searching for problems we should be looking for solutions. A simple google search reveals how the BMW BST works. There are apparently several versions but they all have a screw down post that does not get disconnected from the battery when the main line is disconnected in an emergency. So, there is no real issue getting constant 12V power source if you want it, unless an impact is so great that the battery itself is damaged.