Steve,
If the option is to have one or not get one; I think if a perp is going to choose between a house that has an alarm and one that doesn't he is going to choose the one that doesn't.
As a past business I was ventruing into home automation and ours is ties into the whole house system. It has motion, door alarms with beeps, glass break and window sensors. If triggered all house lights go on, and exterior lights flash.
As mentioned it is also handy to have a one button call, and it's tied into the fire system.
As a bonus, when the kids get to be teenagers we will know when they are getting home or trying to sneek out
