>>1247338 It helps. The bivy is your last line of defense against getting soaked, and, imo, keeps a lot of heat in. If the wind picks up and brings the rain In sideways, you'll be happy for your bivvy. I doubt it would be necessary when camping in deeper woods, but if you're on the moors its a big help.
If you are wary of paying retarded sporting good store prices, just pick up a milsurp goretex bivy. I have the UK army version, which was somehow cheaper here in Canada than our own surplus. Keep an eye out for them,
