I have never slept in a bivy sack but am planning on getting one soon. The rationale:
It replaces a groundcloth (>50 grams) and a bug netting (>50 grams) while adding warmth due to 1) trapping warm air, 2) blocking wind.
If the bivy weighs around 200 g, then it surely makes up for the weight in added functionality and warmth.
What about netting, though? I've studied many bivy designs and found a large spread in the amount of netting used, from an SLD half-netting shelter to some MLD or BMW bivies with no netting at all. How do you breathe in those? If bugs are a problem, those designs would seem wholly impractical. Maybe someone can explain how nettingless bivies are meant to be used.
Are there some conditions when a bivy with netting would become a liability? I'm thinking specifically of the TiGoat Ptarmigan bivy with the full or partial head netting and the MLD Superlight with netting in the area above the head.

