I believe Apple highlighted this when they announced Face ID... Very very very small circumstance. But at least I can see where you are coming from.
Convenience though? You didn't address that.
I'm hanging on to my 6S+ so I can speak about convenience of TouchID.
Convenience is being able to reach in your pocket, and pull out your phone while unlocking it with your finger and not having to angle the phone to your face or even look at the phone.
There are a number of times where one would rather be so inconspicuous as to not have to look down at their phone to even get it ready to use.
That's convenience!