Quoting GP: I think a good balance between security and usability would be to allow fingerprint or iris scan when the phone has been constantly in my proximity but require a pin (password) if the phone is taken away. The proximity could be determined for example by pairing the phone with smart watch.
When combined with a fingerprint sensor, smart lock keeps the device completely unlocked while "triggered" (by being on-body, or close to a trusted BT device, etc), and the fingerprint unlocks it while not triggered. It doesn't ever escalate to requiring the pin/password/pattern. Please correct me if I'm wrong, because I'd like to be.