I doubt many people will know what a Bothy bag is. I read that it was a Scottish term. It is a bag two people can sit in. I have also heard it called a tent-bag and another term in mountaineering I cannot recall. It seems like an excellent survival tool, but wouldn't an ultralight single wall tent be as good or better? Even if you can't pitch a tent on account of conditions, you can wrap up in it and use it like a bivvy or a Bothy.
Jim