Good Q Paxton. No a bivy is not necessary with a tarp and I know people that sleep with just a quilt under a tarp. BUT in most cases it helps. First you can just cowboy camp in the bivy and not setup the tarp — super fast and efficient. This is what Alison and I do about 90%+ of the time in the Sierras. The bivy doubles as a groundsheet, controls gear inside the head area (makes things like phones, hats, gloves, headlamp easily accessible and not in the dirt), and it provides backup bug protection in case we hit a bad patch of skeeters. Second, the bivy shell fabric provides some protection for your quilt from overspray in blowing winds and rain. Finally, it does control drafts when sleeping out in the open, especially on windy nights (essentially providing a large dead-air space around the quilt which significantly increases warmth). That a lot of benefits for +4 oz (Polycro groundsheet 2 oz vs bivy 6 oz).