The usefulness of killing the starter is that it distracts from your other more-effective and less-obvious start-kills. A large part of vehicle security relies on delaying the almost inevitable theft of your vehicle. But like everything else on this site, it's not a solution in of itself; but part of an overall strategy.
There are advantages to this method over the Plug-In, Engine Bay Ignition Kill. This method is built entirely from electronics store parts so there's no need to scour the junkyards. This method is easier to put together for the beginner. Unfortunately, there is one glaring problem with this method on manual transmission cars. You can still push start the car. So I strongly recommend that you use this method in conjunction with the fuel kill. The disadvantage to the fuel and ignition kills is that you can burn up your starter cranking it too long if you forget about them.
Like the Ignition Bay start-kill, this strategy is plug n play, it can be prepared in advance, it's easily reversible, and it doesn't require any cutting of your factory wiring.
Try to imagine a scenario in which a thief cuts the extension cord, potentially shocking himself. The battery charger ceases to function, yet the car battery will last several weeks. Now the thief returns to find that though the car battery is low, the alarm continues to function thanks to the battery backup. Another week goes by. Hopefully by now you have returned from your trip, or at least someone in the house has noticed that your garage light and half your kitchen electronics aren't working due to a blown circuit breaker. Upon investigation they will find the cut extension cord. Failing that, the thief would still have to bypass your car alarm's normally open kill circuit as well as the kill switch. It's not going to happen.