If you want to go by pure odds, the odds are tiny that you'll ever have to use a gun defensively at all, so why do you own one?
You are right the chances of having to use a firearm for defence is tiny. The chances of having to shoot trough a windscreen are smaller again. That's why when choosing a firearm its ability to shoot trough a windscreen would be bottom of my list of priorities. That's why when some say they like the .40 for its alleged ability to shoot trough windscreens better than 9mm i see it as a poor reason to choose a caliber.