I still have bad memories of a very cold night roughing it in a cheapo aluminized mylar bivvy sack. I got damp from the condensation and then the temperature dropped. That was many years ago, but I'm still leery.

However, I can see it as a just-in-case liner for a light sleeping bag. It would add many degrees of warmth, and keep the bag dry. Modern synthetic underwear is a lot more forgiving than the old stuff, so it might not be bad at all.