This old chestnut. True, an alarm unit won't help if they have a truck, but not many thieves have a flatbed truck.
I often read things like "if they really want your car, they'll get it".
However, this completely ignores the other side of the economy of security: if you really want to protect your car, you can. Who would be able to steal my car if it's locked in a safe at a bank surrounded by barbed wire, a minefield, snipers and a SWAT team? No one, and certainly not a thief with just a flatbed truck.
But clearly this approach sacrifices freedom for security, for I would be unable to reasonably drive my car under such conditions.
So the question becomes, how far are you willing to go? Where will your economy of protection reach equilibrium? I would argue that an alarm and a decent immobilisation unit constitute an acceptable level of security without unduly impinging on my freedom to use the vehicle: in essence, these are costs that I am willing to bear. The same goes for a steering wheel lock.
Some people go so far as to avoid driving their vehicles. I am unwilling to pay such a high price (that is, to deny myself not only the benefit of opportunity investment on the value of the car, but also the intangible benefit of owning it in the first place). I am also not willing to let the lowest common denominator dictate what I can and cannot do with my car.