Helicopters are not like fixed wing in ice. Suppose you get just a little ice on rotors, the degradation to flight performance is more then for a fixed wing. Suppose the rotor sheds a little ice - not like a fixed wing where this could be good news. The FOD can impact other important things at high speed - main or tail rotors. Also, due to that shedding you may have an unbalanced rotor which can cause extreme vibration, to the extent of causing damage.
And yet, IIRC(it's been 16 years) the Lynx was cleared up to 30mm of clear ice without heated blades.