The only good solution to stop smash and grab,and vandal theft is cost prohibitive bulletproof laminate($300-$800). As for the alarm a good pager alarm and a concealed carry permit stop actual theft of your car unless u are somewhere u cant carry. But that doesn't work for everone.State laws and such.
Other than that truthfully Yes alarms are usless.I am a decent mechanic in training (R&R in a transmission shop and go to Lincoln Tech) and with no experience in car theft can use a lockout kit to enter a car and retrieve lost keys in approximately 35 secs to a min without any damage to a car. A pro could take ur car and locate and disable most alarms in 120 -130 secs.