I'm with Lori on this one. If the weather is fine (as in clear skies, no wind), you need nothing more than a really lightweight bivy to keep your bag clean. I have a Bibler Winter Bivy (just a bag with a cross-zip) and used it without a tent in winter on snow-worked just fine.

But, once it starts to rain or snow, no way would I want to be in a bivy. Even my SD Flashlight was confining in a storm with two of us in it. Better with one and gear. I considered a bivy, but after thinking about it, I realized, not for me. You could get a Tarptent, a Lightheart (based on the old Wanderlust design), a Six Moon Design or something similar that weighs about the same as a heavy duty bivy, yet has far more room.