If you're that much of a coward, you won't get hit by a wrench anyways because you will give your keys up immediately. But you will force your enemy to exert additional effort.

It works for the same reason locks on houses work against cops or criminals, despite the existence of lockpicking and locksmiths. There are various layers of physical security, and while no layer can prevent an attack absolutely they each increase the cost of an attack.

The system is only as secure as it's weakest layer.

So in a mixed information/physical system like a smartphone why should we allow the weak point to be the information system? To improve the information system we need only to rewrite the software, so the per-unit cost is nothing in the large.