A thief flags you down, grabs your phone and makes you unlock it using your thumb.
A cop opens the cop car door, grabs your hand and unlocks your phone, or even easier, face unlock.
Granted, guns and torture are rather effective as well, but is anyone entirely against fingerprint unlocking?
Also, for the ones that support reset after 10 wrong passcodes, enable that shit. SIM PIN too (mostly for various other reasons.)