This would be great for a compact survival kit, but seems ridiculous to use on a regular basis.

I can't imagine spending 10 minutes stuffing this with leaves every time I got cold, and that's only if it happens to be the middle of fall and all the leaves are dry. If you use something like cattail down, it would take over an hour, maybe several, to gather and stuff all that down. And if the point is to save weight, then you would be dumping out all that insulation when you left camp.

This may be more useful in urban environments, where newspapers, toilet paper, even foam from furniture or car insulation is abundant. In nature you're really limited by season and location for abundant and dry insulation material.