Renurpp,
During certification of an aircraft for flight in icing conditions, a temperature range is determined, where the aircraft is at risk. Within this 'at risk' temperature range, the engine/wing anti-ice/de-ice must be used. Outside this range, there is no percieved risk. In some aircraft types, use of wing anti-ice/de-ice above and below the temperature range can actually damage the equipment.
As far as the MEL application goes, I have always applied it thus;
(1) If you CAN stay completely out of cloud then do so,
(2) If you can't stay out of cloud, then you must stay out of the 'at risk' temperature range. This might mean flying lower or delaying the flight until problem is fixed or WX improves.