Funny - I know more guys who have lost bikes off their top carriers: one in a garage entrance (crunch!), 2 highway speed involuntary "dismounts". Don't know anyone who's had one pancaked in a rear-ender. Though I'm sure it's happened somewhere!
Ouch, yup. Amazingly one only ended up with a bit of rash & a torn saddle (though it was a dh mountain bike). The owner watched in the mirror, horrified as is bounced & kartwheeled behind him. Luckily it bounced right off the road so it didn't get run over by anyone else.
5% is the most quoted figure for the roof rack FE penalty. Though it's really impossible to generalize. It'll be very different depending on the car, the rack style & the load.