Neat product, thanks for the link. Seems like it would make a great liner for winter camping, especially with the double footbox and long sleeves in the 200 weight. With that setup and some long underwear, I might actually be able to use my zero degree bag in zero degree temperatures!(I normally add about 20 degrees to the rating, and I wouldn't use my zero bag below 20 degrees). The sleeves would help with things like reading in my tent without my arms being cold.
The only downside is I would probably wear the bag when cooking/eating, which in bear country would be a bad idea, since then you and your sleeping bag smell like food.