To use Touch ID you will also have to create a passcode as a backup. Only that passcode can unlock the phone if the phone is either rebooted (example full battery drain) or hasn’t been unlocked for 48 hours. This is a genius feature that is meant to set a time limit for criminals if try to find a way to circumvent the fingerprint scanner.
This makes Touch ID just about the most secure method out there. Additionally it would seem to make it near impossible for police to compel you to unlock your phone. All you have to do is power it off, or have your lawyer buy you 48 hours, and then you can see the passcode which is legally much harder to be forced to hand over. Neither should be too hard.
The more I read about this Touch ID system the more I love it.