This is why I use a jailbrake tweak that let's me draw a passcode after initial unlock, but requires a full alphanumeric password on reboot or after 5 ties. I have no clue why Apple has yet to incorporate that feature directly in the OS. Not sure if the fact that the phone being jailbroken is a further vulnerability, but to be honest, I don't really worry about FBI, as much as I am concerned about loosing my phone and having someone find it and open it.