Sparks and embers. And whether it's aluminum, aluminized structure wrapping, insulation, or mylar, it'll reflect the heat away from the side of the cabin for a lot longer than no covering. Toss in sprinklers, and clearing of bushes and other scrub nearby, and I'd say this cabin stands a far better chance of survival than it does without.

It's not going to withstand a 24 hour firestorm directly over it, but I'd rather spend $3k trying to save everything than lose it all because I didn't try.