But that doesn't stop the theiving buggers getting at it.

No but it makes life a little harder for them. They have to force the lock on the boot rather than just smashing a window.

Also, some specialist insurance policies require you to have a boot. For example I looked at getting specialist insurance for my camera gear so I could leave it in the car. Looking at the details neither of our cars were acceptable to the insurers. The Fiesta failed because it was a hatchback, the MX5 failed because of the remote boot release in the cabin (which I didn't want to disconnect). So I don't leave my camera gear in the car...