If someone has the car keys the cars security is irrelevant, unless your cars security involves a retinal scan! Unfortunately that's the way car crime is going these days, it's easier to break in to your house and nick the car keys than it is to try and break into the car and get round the cars security devices.
Trackers are useful, it all depends on the type of criminal nicking your car. If the criminal is planning on exporting your car then it does rely on you realising you car is missing quite soon after its stolen otherwise it could end up in a shipping container before the police intercept it then the signal will be very hard to track. Some car criminals anticipate the use of trackers on high end cars and carry signal jammers with them. But if your criminal is just planning on re plating it then selling it on you stand a chance of getting it back.
Lots of insurance companies insist on them on high end cars and lots of heavy plant machinery have them as well. They've realised they do significantly increase the chances of getting the vehicle back but it is by no means a guarantee.
Only other down side I found when I had mine is the subscription fee for the tracking company to monitor the device. It was an annual fee but you can pay a one off lifetime subscription instead.
Trackers have come a long way you can even track your car on google maps once its stolen to see where the little darling are taking it and how quickly they're getting there! lol