The purpose of the carby heat is to clear ice using hot air. If the heat is only partialy on it may melt only some of the ice and cause icing problems further down the carby (Cold water re-icing)
With only partial carby heat you cannot tell the temperature of inlet air you are supplying.
Some aircraft have a carb air guage (forget the correct name for it, but i do recall it being fitted to a later model C182 I flew). Only in this case partial carby heat may be used as you can ensure inlet air is above the required temperature to melt all the ice.