I use the power button to lock the phone (turn off the backlight), to mute incoming calls when the phone is in my pocket, and to take screenshots. It would suck for me (and a vast majority, I think, of people who do the same) to have to take off the case with a "special" screwdriver.

A better option would be to prompt the user to enter their passcode before turning the phone off, if at all the user has set a passcode.