Someone posed this question to Honest John in the Telegraph motoring Q&A's. A BMW had had four windscreens in something like eighteen months, none of the breakages due to impact. HJ replied that the windscreen on a BMW is part of the structural integrity and if the goo isn't left to set properly before moving the car, the screen will fracture when the body flexes, as they do when going over speed-humps, pot-holes, etc. I would imagine this is true of some other makes and models, not just BMW.