The bikes ridden long distances on the road do, the ones used primarily on trails don't. Now that I've had them, I really like them. Useful for more than just traffic too, if I am in lead I can easily keep an eye on the fiancee and make sure I'm not setting too fast a pace, much nicer than sitting up and loking over my shoulder every minute.
Biggest problem I have is getting the handlebar mounted ones to fit well on things like trekking bars and drops with bar end shifters, it takes some time to get something to work. Second biggest problem I have is I am rough on my helmets, so a helmet mounted one would never survive me.