I agree with others here. and ultralite tent can be almost as light and it gives you several advantages. If it rains you still need a tarp of some kind to cook under, change clothing, sort through your pack for stuff and generally have a place to sit. You don't want to be in that bivvy more than you have to. It is nice to sleep in sometimes, but you need to get out sometimes too. I use a bivy when it is cold but with zero chance of rain, like high in the mountains above the treeline last summer. A couple of times it was very windy and the bivvy worked better than a tent. But in rain you would be better off in a UL tent or under a good tarp. IMHO
Jim M