The Filament Pullover is the lightest insulating layer I own. For its weight, it keeps in an amazing amount of warmth. If I'm backcountry skiing, I'll stuff in my pack. It takes up almost no room and will provide an extra layer of protection against the cold in case of an emergency bivy. I also have worn it as an insulating layer, at least until I heat up. Given that it weighs so little, there is no reason not to carry it in your pack. I imagine it would be useful as a jacket when camping out on cool summer nights, for example.