> Watching someone key in a PIN and recording it, then swiping the phone is easier than building a 3D printed color model of someone's face. Not to mention that having the biometric unlock sitting on top of a PIN means that there are many fewer chances for the PIN to be observed.

With how cheap video surveillance is these days, any PIN that you've regularly entered on your phone in public is probably recorded on video somewhere.

So is your face, of course, but like you said that's much harder to reproduce.