Belt and braces I guess, if you are descending the temperature will eventually climb to the level that you will start to get ice problems. As for the bit about turning it on in the descent if icing conditions are anticipated I don't know anyone in our company who operates it like that, what about other operators?. We just turn it on when we are about to enter cloud or rain, doing it the Airbus way I reckon you would have it on most of time in the descent in Europe.