Personally, I'd prefer a permanent footbox since it's unlikely I'd be using it as something to wear. Especially around a camp fire where embers would end up burning little holes in it.
What would be really great would be if it was shaped in a way that it wrapped over the shoulders well to keep drafts out. I'm thinking of a deep head notch cut into the top edge.
Another idea would be to have the 2 layers to it. That way I could use just one for summer temperatures, and the combine them when it gets for real cold out.