Even on the track, they are of extremely limited use in terms of performance gains. The main advantage is that ones will a hollow 'void' in them provide a convenient place for a camelbak resevoir for endurance racing, or, if you're into that sort of thing, telemetry kit. On the road? 100% show/fashion. Still, no harm in that if you like the looks. The cruiser crowd don't exactly dress the way they do for functionality.
