Apple is driven by two sometimes complementary needs. The first is convenience; the second is security.
Convenience: Holding the phone up is more convenient than touching the sensor, which is more convenient than entering a number.
Security: Thumbprints are interesting because they fall into a strange legal loophole. Legally, the government can not compel you to give up things that are in your head, such as a passcode or a combination to a safe, because the Constitution prevents you from being forced to testify against yourself, and the Supreme Court has ruled many times (going back to the 1800s) that compelling you to give up something in your head is compelling you to testify against yourself.
However, the courts have long held that you can be forced to give up your fingerprints. Those precedents were set before biometrics, but they've been interpreted to mean that the police can not make you give up a passcode to your smartphone but they CAN make you use your fingerprint to unlock one.
Apple is a company that is strongly on the side of personal privacy over police action. It's not clear that a facial scan could be required by police to unlock a phone.
It will be interesting to see how this plays out.
As for facial injury, you handle that the same way you handle getting a cut on your thumb: if the scan doesn't work, you just type in your passcode.
Edited by tacit (02/21/17 07:15 PM)