In my opinion the shelter aspect is by far the biggest value of a bivy. If you are wearing reasonably adequate clothing and can improvise some insulation from the ground underneath you (a foam pad, for example), with a bivy you can make it through the night. You may not sleep much, but you will survive.
Exactly! And that's why they're worth their weight in gold in cold climates.
As a side note - one of the things I'm curious about is how well my various daypacks could work as improvised insulation. Only one test sample so far. My biggest day pack (55 liters) worked quite well - but that pack has a full size back length, so with a little improvization I can lay flat. Test conditions: +5C, dry, little wind. Clothing: one thin layer of wool under the shell layer, augmented with a fleece jacket. Bivy: Jerven "fjellduk" (the Jervenbag original,
http://jerven.mediabook.no/3/ (brochure in flash format), which is uninsolated, vapor barrier with the typical shiny "radiation reflection" inside.
Results: 4 hours of good, sound sleep - then I woke up and realized I couldn't turn into my favourite sleeping position. If I had brougth a pillow I would have stayed through the night, but I crawled back inside to get some more fresh hours before work next day. I was surprisingly snug, warm and comfortable - and not really too bad and clammy. No doubt I could have stayed much longer and still be fit for fight the next day.
I want to test this further with different configurations and temperatures, but real life kind of gets in the way...
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