I rented a room on the second floor of a townhouse for three years. It was close to work, rent was reasonable, and close to a bunch of amenities. My room overlooked the street. Across the street was a neighbour with a car parked in the driveway. At least once a week between the hours of midnight and six in the morning, our neighbour's car alarm would go off (cats, wind, trucks passing by, snow, who knows). The nieghbour would get up in the middle of the night, find his keys, and then turn off the alarm . . . usually after about five minutes.
One night his alarm went off at 3:00 and stayed on until 4:00. My fiance somehow managed to keep me from going outside and throwing a brick through his house window to get his attention. I'd actually describe myself as a pretty peaceful guy, imagine what would happen if I was a raging lunatic. Or my fiance wasn't there.
Not only do car alarms not protect your car . . . they may very well endanger it.