Hi Baobab,
I cant answer anything about the ATR, but the reason for holding clean is when you have flaps extended you need to have a nose down attitude to fly straight and level, this in turn means you are exposing the top surface of the wing directly into the relative airflow. When you are in icing conditions the ice will build up aft of the deice boot and lead to a stall, In this case the stick shaker will not activate prior to the stall