I think the alarm is deterrence for inexperienced thieves... Someone who knows what they're doing will disable your alarm in less than 30 seconds.
I have a cheap old alpine alarm for deterrence, and I have two remote(aka far away from each other) and novel(they target 2 different systems) killswitches that I really put my faith in... that'll stop most any thief looking for a quick hop-and-run....
Though, I'm pretty much ****ed against a good thief in a tow truck... But most everyone is