In the Sierra I have done trips of up to 14 days with only a bivy- just a basic MSR bivy, no hoop or anything else. I have done shorter 2-3 day trips in the Rockies IF I have a good weather forecast. I am small, so a bivy fits me fine- I do not feel tight or claustrophobic at all. In fact, I put my backpack inside as a pillow. Shoes go at the very top of the head part. I sometimes take a small 5x8 tarp and set it up on my trekking poles just for head coverage. That way I do not have to zip up inside during rain. Also, you can tie up the top of the head part on trekking poles.
I can see where bigger people do not like them. It does take some getting used to them.
1) weight (basic bivy is only 1 lb 4 oz)
2) you can cook and eat in your sleeping bag! Just prop up against a rock.
3) no set-up
4) fits in very tiny spaces- great when terrain is very steep and you can only find very small ledges
5) you can sleep on the top of a flat rock
6) you can immediately jump inside during a brief storm
7) low profile wind-proof --- excels on mountains - in fact that is what bivys are intended for- spending the night on a multi-day climb or on a small ledge
I have a few photos of set-ups of a basic bivy. I have not inserted a photo for some time in any post, so I need to figure out how to do this. If I can, I will add the photos.