It seems to me that a tent without a fly would weigh more than a bivy sack. It would also take up more room in your pack and take more time to set up. If you guessed wrong about precipitation, you could be really hosed (i.e. soaked). My bivy was all the shelter I needed for winter ascents of Boundary, White Mountain Pk., Split, Langley, Russell, and Middle Palisade. If I’m going to save weight by leaving the rainfly at home, I’ll leave the whole tent at home and bring a bivy.
