Alarms/Immobilisers are all well and good but is that not why things such as Tracker are a good thing.
No warning stickers, thief takes car, you wake up, phone tracker and they tell Police where your car is (hopefully with a load of other stolen vehicles that can be returned to their owners).
Granted, your car could be in bits if you don't notice for a few days, but at least there's a chance of recovery of the car.