I went down hard on my 30 year old steel bike and dented the top tube slightly (against the raised curb), scraped up the handlebars, right pedal, and myself pretty good. Had that been a carbon fiber frame, I think it would have been toast.

Everyone wants lighter, lighter, lighter... but really how light of a bike do you need. 20 lbs is plenty light. We're not pros - most of us are not racing. You're supposed to be exercising anyway.