Dehumidified Defrost needed?

Dehumidified defrost I think is more of a "luxury" allowing you to defrost the windshield without needing to turn up the air temperature (although more of a necessity in warmer humid climates like where @Demonracer is). The process of heating air in itself lowers the relative humidity, that's why the air inside your home gets dry in winter when it's cold outside. Otherwise like mentioned it would be impossible for defrost/defog to work on vehicles without A/C.

If it's below about 45-50 degrees outside you'll probably be wanting to use the heater anyway.