Personally, I think dressing properly for the environment is much more important than having a sleeping bag. I've spent many a night sleeping without a sleeping bag, even waking up with snow on the ground the next morning. I see bums in NYC in winter that sleep in just they're clothing (though they wear enough layers of clothing that they look like a walking sleeping bag grin). A sleeping bag, IMO, is secondary to a proper shelter set up and proper clothing for the situation.

With that said, I keep a Thermo-Lite Bivvy in my BOB (as well as a few large garbage bags). The Thermo-Lite isn't a true sleeping bag, but I think it's plenty in most situations when you just need that little bit more either to keep you comfortable, or to keep you alive.