The reasons for all the above are obvious - the alteration of the aerodynamic properties of the aerofoil with ice formation on the wing (shape & skin friction). But one other consideration that needs to be taken into account is the addional weight that any ice formation adds to the airplane. Granted that ice on the fuselage may not produce much adverse effects on the lifting capability of the wings, but large buildup of ice over the airframe as a whole could significantly increase the airplane weight... in unmeasurable/un-quantifiable increments. If in doubt, de-ice the whole thing.