Just to clear up a possible confusion here, the 737 2/3/4/5 temperature requirements refer to ENGINE ANTI-ICE (not airframe) and I have always understood the +10 to allow for intake pressure drop.
For info the only time I have seen significant airframe ice at -40 and below is in strong mountain wave or in the top of an 'active' where extra moisture can be carried up to those levels (I personally recommend (and use) ENGINE anti-ice at -40 or below THERE for that reason!) The amount of engine ice normally accumulating at these temperatures is judged to be insignificant, but at idle there might not be enough centrifugal (am I allowed to say that?) to throw any ice off, hence the req for descent.