The foil bags are a very reasonable aid to wilderness camping as a "supplemental" survival tool. They would be fine for draping over a sleeping bag and increasing your heat rentention. The point here is that you already have adequate body heat, so the foil bag can be of some assistance.

But the concern is that inexperienced people who go to a camping store might mistakenly think that these foil bags alone could save your life in a really cold environment.

Imagine a serious situation ... where you are thrust into a very cold environment (freezing) and your clothing is inadequate (e.g. most of your clothes are wet and freezing cold, and you cannot dry them until the next day). Imagine also that it's late in the day, so there is limited daylight before darkness.

Under these conditions, survival for the next 12 hours depends critically upon finding shelter and preserving your body heat. You don't have time to find food or water, and may not even be able to find fuel for a fire. Therefore, you absolutely need a good sleeping bag and preferably a bivy sack as well. With these essential items, and a survival tool (good knife or tomahawk or machete)you might have much greater hope of improvising some form of temp shelter, getting into a warm sleeping bag, and living to the following sunrise.

This logic leads you to realize how important it is to have a few essential items that you can really depend on - to ensure you can stay warm and get some sort of shelter. With adequate body heat, people have a much better chance of keeping a positive mental attitude. Without these vital things (warmth and positive thinking) - death can come quite quickly.

Something to consider.

Pete