I think having a glass breaker is a fantastic idea and adds an additional use to folding knives other than cutting.
There have documented rescue cases where people have tried to poke the knife point into the window to break it. In these cases, the point, pivot, blade break, hand slips or the blade slips because the force is not at a perfect right angle. It will probably totally impractical and dangerous to do such a thing with a knife.
A small glass breaker on the otherhand can tackle most automobile windows (laminated or not) and create the cracks and the opening you need to open the window.
If you keep a knife in the car its a great idea to have a knife with a cutting hook (for seat belts) and a glass breaker. So for rescue purposes such a feature is a life saver.
Thanks to all those manufacturers that decided to include such a feature.