The main thing is deterrent. As I stated before, I don't care what you do, if someone wants your sled, they are going to get it. I've had a sled stolen from behind a chain-link fence, that was locked down to the trailer, and also to other sleds via cable.
The dudes that stole it, drove into the ditch, cut open the chain-link fence, cut the locks, and picked up the sled off the trailer, and loaded it into the back of the truck, and drove off.
Locks, cables, fuel cut-off, all these things work in conjunction with each other to SLOW THE PERSON DOWN. Theft deterrent, that is all they are.
Pay your insurance premiums, do what you can to deter thieves, and hope it doesn't get stolen.
While I like my sleds, neither of them are irreplaceable. Insurance premiums are horrible to pay, but are worth every penny after a sled is ripped off.