That's so silly - physical access to the machine doesn't mean anything per se!
What if you can't take the machine apart inconspicuously because the case is sealed. What if you have only 3 minutes before someone else comes by? Security is not black and zero at all.
One can easily even use an AVR that'll replay the keypress sequence over USB (posing as a keyboard) on a button press. This is something completely different than taking the machine apart to clear CMOS or whatever.
BTW, can you have UEFI trusted boot with GRUB, or do you need coreboot? (Yes, there are people other than Microsoft using it, e.g. when selling appliances - think vote machines or gambling terminals.)