Not me. I prefer the head room and mosquito proof tent. I have gotten soaked just trying to get in and out of a bivy and changing clothes is hard in a bivy. I have used my goretex bivy - Western Mountaineering 21 ounces - exactly once, in late fall in Yosemite in Tuolumne meadows and it worked great - no mosquitoes. no rain or snow, and back then I had n comparable weight tent.
I would never use a bivy under a tarp, thats the worst of two worlds.
Jim
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These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.