Range is range, your space heater idea is just a less useful plug in hybrid. Spending less money on batteries to add another system that you use a few times a year results in a generally less useful car.
It depends on the relative cost of "N hours of heating in the form of more batteries" vs "cost of a burner heater capable of N hours of heating". If the costs are even nearly comparable (if they aren't 10x apart) then I agree with you there is no point.
The heater also can't be as complex as a plug-in-hybrid, requiring expensive serveice etc. It needs to be cheap, dumb, optional (so you only waste the cargo space on extreme long journeys and in cold weather) etc.
What I'm picturing isn't a complex system. It's a camping burner with a hose to the car's ventilation system. Think < 1cubic feet and sub $1k.
3-4h 1kW heat sounds reasonable. If that's less than say 3% of total range, or less than $1k extra battery then I'm confident it's a sufficient solution.
Model 3 users report using about 240 to 300 watt-hours per mile. So I think that means a 1KW heater for an hour is equivalent to about 3 or 4 miles of range.
So a 1KW heaster for 4 hours would lose you 16 miles of range.
