I agree with you that nothing is full proof, but I've seen security mechanisms that are so compromised that they don't stop thieves at all any more. For example, a steering lock column might have been a minor deterrence when thieves were actually crossing wires to start a car. Once they moved to attacking the ignition cylinder or the column, a steering lock column's protection became completely ineffectual. A hologram deterred credit card counterfeiters when they first came up, but now you can't buy a counterfeit credit card blank which doesn't have the hologram in place and looking perfect.
You cut through ninety percent of the strap when the person is standing still and then take off running and the strap breaks. If the thief moved from a sharp blade to a cutter as his default tool, the Pacsafe would be useless. If the side panel isn't protected and that is where people normally cut, the bag isn't protective.