Where I live, cold spells unfortunately tend to coincide with very low winds.

And yeah, adding a supplemental resistive heater to a heat pump is certainly a lot cheaper than adding a separate combustion based backup heating source.

For some reason, we don't really use propane over here, except for running grills in the summer, or for boat stoves etc. Fossil fuel heating for houses tends to be fuel oil, though that is on the way out.