$4.00 Car Alarm

Is perception of an alarm system good security? Or just another part of a layered defense?
For instance, for US$4.00, you can purchase a blinking red LED box from Radio Shack (part number 276-299). It looks like this:

Add two AA batteries, mount it in your car (some velcro tape is included), position the LED, and flip the switch.

Voila! A car alarm system!

Or at least one that will make a potential theif look twice.