Here in Quebec, home heating oil is approx $1 liter ($3.90 US gallon).
And that electricity is cheap here (dual enery DT rate):
Warmer than -12C(10.4F) the price is 4.3 cents/KwH
Colder than -12C(10.4F) the price is 20.4 cents/KwH
Most of the time it is warmer than -12C(10.4F) therefore we want to electric heat as much as possible.
The defrost cycle of the heat pump uses oil to speed up the process. We don't want that!
In my case the heat pump puts out approx 38C(100F) heat. But it can drop to 30C(85F) - 32C(90F) by the time it gets to the bedrooms (becasue it's a long skinny house and the duct is over 50ft long). So the plenum heater would help the heat pump (but not when colder than -12C(10.4F)).