I'm with Sean for the same reasons.
Maybe we need to use a new term, "bivy" for the standard waterproof bivy that people seem to not like at BPL and use "UL bivy" for the ultralite, water resistant, highly breathable type.
There are lots of comments about bivys being constricting, damp, stuffy, ...
A UL bivy usually doesn't suffer from the comfort issues those old military goretex bivys do.
I usually use my bivy like Sean, as a way to cowboy camp to keep bugs off and any light spray rain, dew and dirt off my bag when I cowboy camp
I can then set up the tarp when it might rain.
Actually, I usually hike in the North East where it always seems to be a 30% chance of precip. So the tarp is usually pitched, but pitched high for ventilation.
I even use the bivy in cold weather. It adds warmth by helping to reduce drafts and having the bug net suspended over my face helps reduce the cold nose issue. It is worth the 7 oz for year round hiking.
Also, my my main bivy, an SMD Meteor, is roomy. I flip flop all night and never feel cramped.
My other tighter, full coverage Oware Drawstring bivy, seems just as comfortable. It is light and moves with my bag when I squirm from side to side.