My understanding is like has been stated. If someone is intent on stealing your car or it's contents, they will attempt it whether you have an alarm or not. Some professionals will either be so experienced with their crimes that they can commit them and not be hindered by the triggering of an alarm. They can have what they want and be gone before they can be caught. Others may know exactly how to get around the systems.
From talking to friends in Law Enforcement, the most valuable part of a security system, whether on your car or your home, is the sticker on the door. While that won't stop the most professional and determined it will deter most attempts that are probably crimes of opportunity or amatures or desparate types who just need a few bucks for a fix or a bottle of booze.
Largely it might depend on where you live. Do you live in a fairly safe area where most car thefts are serious thieves who intend to chop up your car or put it into a cargo container and ship it to Poland or Mexico.... I have seen news stories where stolen SUVs were spotted on the streets in Eastern Europe. The Illinois License Plates and dealer frames were still on them. Then there are places like in cities where cars will be broken into and ransacked for whatever can be found in the glove compartment etc. The car itself will not be taken but your camera or computer stolen out of the trunk. These are people who might be detered from an alarm sticker and will go to the next, UNPROTECTED, car rather than taking a chance of setting off an alarm.
I am considering an alarm for a few reasons, First, I live in the former area where nothing I do is going to stop a determined and experienced thief. There aren't as many of those people so I have little fear of having my car stolen. However, I do carry some expensive items from time to time and want some protection from a smash and grab type situation. I am thinking of the Viper Responder 791xv here. It has a pager function so if someone does attempt to break into my car, and I know there is something in there, I can get a readout of whether a shock (hit) was recorded, whether a window was broken, the door opened? Hood Opened? etc. Knowing the situation can alert you to call the police yourself, from your cellphone or to go out and invstigate a possible hit and run accident. You migth see the guy driving away. While an alarm might not cause others in a parking lot to run to defend your car, my experience is that human nature is such that people do look up when one goes off. Therefore, if someone does smash your window and grab something out of your car you might be lucky enough to have a witness or two when the police do arrive. Another reason for a pager alarm. Don't rely on someone else picking up their cell phone to call. However, if you get a readout that the window sensors were triggered, indicating broken glass, and a door opened, as if someone has gotten in to open your glove box or storage spaces.... Maybe your tailgate(trunk) was opened. These together would cause me to grab my cellphone while running out to my car.
As good as an alarm is, some other things are more important and maybe more valuable. I would recommend getting the cargo cover. It is fairly expensive but Extremely easy to install. It goes into the back, behind the rear seats and pulls out like a window shade, and clips into the frame of the tailgate. With the dark windows and the cover, you CAN NOT see if anything is in the back of the car. Don't leave valuables in plain site. Hide things under seats or stow wallets, jewelry etc. into the armrest glove compartment or overhead storage. Or don't leave valuables in the car at all. If you have to, OUT OF SIGHT OUT OF MIND. Don't make things easy for a casual thief. Also, don't park in secluded places. Park where it is well lit. Lock your car....