How good a camera would Touch ID make?
Apple states in this Apple Support article:


  The steel ring surrounding the button detects your finger and tells
  Touch ID to start reading your fingerprint. The sensor uses advanced
  capacitive touch to take a high-resolution image from small sections
  of your fingerprint from the subepidermal layers of your skin


This tells you that a) the sensor only starts reading when being touched and b) it's capacitative, limiting the typical detection distance to the sub-millimetre range (capacitance is proportional to 1/distance).