For me it is an essential, but then I camp all the year round and not phased by extremes of climate.
It takes a few minutes to warm the Cali, it has immense reserves of heating capacity, it can be programmed to come on when you want it to, it can be remote controlled (wake up in the middle of the night freezing, press a button, two minutes all snuggled up and warm without getting out of bed).
It is independent of needing gas, or looking for electrical hook-up, economical, as long as there are reserves of diesel in the tank you have cold weather capability, and you will need certifying if you go into wild remote region in bad weather without reserves of diesel anyway.
Horses for courses but for me it turns the Cali into an all year round wild weather camper.
It does not need to be just for extreme climates. This next week will prove it's worth, and if I manage to get away to Devon I will be very grateful for it. Yes, I will have hook-up, I can carry an oil-filled radiator, but why lumber myself with all that, and the thinking and planning time, when I can just be away and happy and warm and snug.