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OK, I’m going to blog-post here… bear with me as I’m a little long-winded. I want to use this as copypasta for the next bivy thread because I think it’s worth repeating. If you don’t want to read it all, I’ll give a TOC:
Post 1 = What is a bivy
Post 2 = Why do they suck
Post 3 = Why are they still good
I've said this before, but general use of these kinds of shelters is often rather misguided. Bivy sacks and bivy tents have one main niche use by themselves, and they do well in that niche. Outside of their niche, when applied to general usage, they are underwhelming in every regard.
The bivouac’s main use is in mountaineering situations, where level ground to pitch a tent might be hard to come by. For instance, think of mountaineers needing to rest for half a night before continuing their climb in the early morning, or mountaineers needing to set up a shelter to get out of the rain or snow. Those kinds of people in those kinds of situations have found that when it comes to shelters, the simpler, the better.
A mountaineering bivy need not have tent poles, or stake tie-outs. Because of the intensity of mountain weather, and the scarcity of trees at higher elevations, being low to the ground is an advantage. Your sleeping bag and pad are what keep you warm. All your shelter has to do is keep the weather (the most common of which they face is is often snow, wind, and blown-snow) off of you, and that's what these do in an ultra-minimalistic way.
Pic related is the kind of situation, or setting, in which a bivouac-style shelter outperforms all other options, like tents, tarps, and hammocks (ignore for a moment that the "bivy" in the picture is actually more like a tent). In terms of ease of set up, not needing to be extensively staked out, and having a low, mountain-worthy profile, this style of shelter is ideal for the mountains.