I carried a bivy for off season AT hiking generally when it was colder and possibly damper. I rarely used it outside a shelter but found that it gave me 10 degrees additional rating for temps by sliding my sleeping bag in it. I agree in rainy situations they are better than nothing but its almost impossible to get in one when its raining without getting the interior wet. Mine has a goretex top panel, realistically it weighs as much or more than the current lightweight tarps. If the weather is wet and windy the bivy is going to work while a tarp is very prone to the quality of the setup and the underlying ground. I have used mine in mud puddle, not by choice but it was the only flat spot around. It got dirty and I had to wash it off the next day but ti didn't keep me a lot cleaner and dryer than using a tent in the same space. My bivy has a hoop over the top and sort of a small vestibule above the head but when all sealed up it tough to avoid condensation even with the goretex.