The alarms did save one of my cars once. Back in the old days, I had a Buick Grand National. I ordered the car with a factory alarm, and later added an Alpine alarm with battery back up, hood lock, glass breakage sensor, tilt sensor, shock sensor, the whole works when I installed the sound system.
I parked my car in a self-park hotel parking garage once, but I forgot to arm the Alpine alarm. When I walked back to my car, I noticed from the passenger side the door was unlocked. I thought, odd, I thought I had locked it. Upon inspection, I found someone had popped out the lock cyliner on the driver side and unlocked the door.
The thief might have noticed my Alpine alarm LED was not blinking, and decided the alarm was not armed. He (and I) had both forgotten that the factory alarm always arms when I lock the door with the power lock button.
I had a $1K+ head unit and an equalizer in the car. But despite the fact they opened the door, the alarm scared them away as it would have taken only 3-4 minutes to steal the stereo. And probably not much longer to pop the steering column to hot-wire the car (which would not have done much good as the factory alarm disables the fuel pump, and shorting the ignition wire to crank it will never get it to start).
Long story short, the car was saved. I filed the police report, but of course nothing came of it. I always armed both alarms in the car from then on.
It is true that most people ignore the car alarm these days, but for the thief, he probably will not want to take the chance just in case someone does look in his direction.