There are a few factors to consider before deciding whether auto glass insurance is right for you. For one thing, the coverage may already be included in other parts of the policy. Collision insurance covers windshield breakage in the event of damage that was not your fault, which includes a rock thrown up by another vehicle. Comprehensive insurance covers other types of breakage, such as vandalism or burglary. Since many dealerships require you to carry both collision and comprehensive insurance if you want to finance the vehicle, purchasing auto glass coverage is unnecessary.
You should also compare the cost of the coverage with the actual replacement cost. If the replacement is near or less than your deductible, filing an insurance claim accomplishes little more than helping drive your premiums up. Unless the glass replacement coverage is going to be beneficial to you in the long run, you probably won't need to buy it.
On the other hand, if you live in an area where vandalism is common and windshields are a favorite target, having auto glass replacement coverage can save you money in the long run as the policy deductible only has to be fulfilled once. After paying a majority of the first replacement, the following ones would be covered by your policy.