You must be determined not to "get it".
No, I am determined that there are better things to be doing with my time and resources that will be better suited to my defense than pressing a panic button and hoping there is a remote chance that it will have a beneficial effect.
You are right. It might work. Of course, all the time and energy I put into believing that a car alarm might stop a home invader from killing me might be time and effort best put into other things like getting a better lock on my door, having a better alarm on my house, buying a dog that barks, etc.
Sure you might just happen to be surprised by such a neophytic criminal that he knows nothing of things like response times or what alarms mean to the general public or the police, but is somehow bright enough to best all of the defenses that you have an place as a person concerned about self defense. It could happen. The engine of a plane could fall of and crash through my house as well.
If you are betting on the remote chance that a car alarm might stop a home invader in your home, then you have already made some fundamentally bad decisions about self defense.
Put another way, if you think that the 2-5 seconds of pressing the panic button on your car alarm to get it to activate, after you find it in the dark or while being attacked, is a good investment of your time and resources for the chance that it might have some minimal impact on the aggressor's behavior instead, that is pretty silly. It is really silly when you consider that during that time you could be doing something like actually speed dialing 911 (that might bother neophytic attacker to know that you have called 911, it could happen) or doing something actually proactive in your self defense like manipulating an actual weapon for those 2-5 seconds that really could make a difference in your defense that you can count on instead of hope beyond hope that the effort might possibly beneficial.
For example, the MAAT-7500 requires a 3 second press...
http://www.scribd.com/doc/187699/MAAT-7500-Car-Alarm-Manual
Klimaco doesn't give a time, but through their complicated instructions note that the panic alarm works by "press and hold" so that you can not only scare away people in a parking lot, but find your car.
http://www.klimaco.com/caraudiopages/car_alarm_help.htm
Now some alarms do allow for immediate noise generation if you press the correct two buttons simulaneously. If they are on the same side, the process can be difficult or impossible to do readily with just one hand (my Toyota, for example). Some are more user friendly with the buttons on opposite sides, assuming the press the correct buttons (Subarua
http://www.cars101.com/subaru/keyless.html).
Of course, few folks buy dedicated panic button fobs (which you can) and so are left with the multiple buttons on their car alarms, that potentially will turn off the system if the wrong buttons are pressed or will reactivate the system such that the system goes through its brief shutdown or setup sequence before it the panic alarm will initiate.
Of course, the folks at Snopes have their own ideas about this topic. Sure, somebody is breaking into your house you can do the whole panic alarm and maybe it will work. Then again, turning on a light does the same thing in a fraction of the time and tactically offers several more advantages.
http://www.snopes.com/crime/prevent/caralarm.asp
Most folks just don't have enough brains, hands, and abilities to be doing all the things actually necessary for their defense as well as trying to initiate all those things that might have some remote possibility of helping as well.
It is a car alarm. Nobody cares. Of course, if you aren't going to do anything actually helpful to try to protect yourself, then you go ahead and push that button. You push it like your life depends on it because you are committing an undue amount of your available resources that could otherwise be protecting your life.