Remember, if the burglar doesn't know that the window won't break, he or she will find out by trying to break it. If they can't get in, they can't steal anything from inside, but you'll still have a window that needs to be repaired. Maybe the insurance will cover it, and maybe the deductible will make it your responsibility. Bars over a window or door make it obvious that they can't get in easily. Most burglars won't vandalize a house just because it defeats easy entry. It makes noise to break glass, and it's a crime without a profit. Some burglars do vandalize, but that usually happens after they get in.