it's kind of like that joke..
two mates are in the bush and they see a grizzly bear coming right for them. one bloke turns to another and says in relief "it's lucky i brought my running shoes!".
his friend replies "what good is that? the bear will outrun you anyway". and the original guy says "that's okay, i only have to outrun you."
the more immobilisers, the more time it requires to steal a car. the more locks on your front door, etc. etc.
i just have to make my car the least appealing in the car park to try and steal, so the better the alarm, the less chance of theft.