Any time I plan on sleeping overnight obviously I would pack a sleeping bag and a bivy. The reason why I have always prefered this method is it that it has multiple uses. Ponchos can be used as ponchos, bivys, tarps, and tents. A poncho liner with a head hole sewn in the center can also be used as a tarp, tent, and coat. When I have the money I plan to put one together and cinch it to my Hydrastorm for a light summitting pack.