I'm not a bivy user, but from everything I can gather, a bivy alone without a tarp is a very poor fit for a trip with lots of rain, while a bivy with a tarp is very little different in terms of weight than a ultralight or lightweight tent. You can change clothes inside a tent far more easily than inside a bivy bag, and when there's lots of rain you will need to change clothes because your clothes will be either damp or wet.