The way I see it, there are 2 types of car theif, the oppertunist and a professional.
The oppertunist will break in to your car and steal whats in it, or want to joy ride, along with the obvious alarm / imobiliser, a good deterant is to hide a kill switch to the ignition or fuel pump or remove an HT lead or something - this should render the car undrivable.
The professional car theif will be much more determind to get a car, and despite complex alarms / imobilisers, chances are they will get your car either by towing it or putting it on a flatbed truck and not even try to start it there and then. This is where a tracker comes in as the car will probably be taken somewhere to be stripped for parts or exported from the country