In reply to Fredt:
I'd suggest that sometimes the little extra weight is simply worth it. After spending hundreds of nights in bivi bags in the Army. One thing is for sure, you can't admin yourself in a bivi bag. As soldiers we used a tarp equivalent (basha). If you only have a bivi bag, then you end up changing your socks in the rain, which defeats the point as they end up damp anyway; and have to cook in the elements.
The human body naturally gives off moisture, so you will always end up with some condensation. As posted above people tend to use a synthetic bag with a bivi bag. A small tent allows you to use down which gains back a little of the weight saving.
I swear by my little TNF solo 12 for UK use; but there are better products out there now. Although I use a bivi bag in the alps, when I'm usually expecting good weather and spending fewer nights in the hills.
Matt