Your car comes with a factory alarm. These alarms don't work on impact. Only when they open the doors and try to start the car or steal the radio.
Personally, I have the same belief. If someone is determined to steal or vandalise your car they will do it. Even when the alarm sounds the damage has already been done. However, they are good to scare them off (sometimes), and preventing them from doing more damage.
They are also good for the keyless entry.
Anyway, I am pretty sure that someone can setup the current alarm system to your specs. Make sure the impact sensor isn't too sensitive. Most of the shops know how much force is required to brake a window. The rest are personal prefrences like keyless entry, automatic locking (I'm not a big fan), etc.
I suggest going to a proffesional and have him check it out. This way you can avoid someone cutting more wires in the car to put a different alarm.
They can even change the siren if you find yours annoying. Personally, I like the Alpine sound. If installed properly you'll probably never have to hear it.
One more thing that many people overlook is how quickly aftermarket alarms can drain your battery. Most of them can drain the big 100Ah Mercedes battery in about five to seven days.