Recently our car was broken into, despite being parked in what I thought was a relatively safe place. While getting the broken window replaced, I learned that a key consideration when designing car windows is that they shatter safely, so as not to injure passengers. Car windows are not primarily designed to keep the crooks out; in fact the police officer who inspected our car was surprised that the burglars apparently needed to hit our window more than once in order to break it.
A corollary is that car windows are easy to replace. If you design something to break, you might as well make it easy to swap. Many car windows are apparently held in place by just two little hinges (as is ours) and the entire replacement process takes 15 minutes.
What this means is that you are probably better off paying for optional “glass insurance” than you are for a better car alarm system. If someone wants to break into your car, the alarm is not going to stop them. It might even lead them to damage the door or other parts of the car that are more expensive to repair than the window 😉
In Australia, even “comprehensive” insurance packages do not usually cover damage to windows or windshields. In our case this is an optional extra that costs around $60/year. It turns out that each window costs around $200 to replace (the deductible is typically around $500). So at least to me, this seems a worthwhile extra to pay for.