For riding long distances at highway speeds, I'd consider them nearly essential. I remember when I was riding my naked cruiser for a 3 hour interstate highway ride just how physically exhausted I was at the end. The buffeting of the wind against your chest means you have to hang on tighter to handlebars. If you crouch down you get less wind, but now your body weight is shifted to your wrists and back, so up you go again to get buffeted by the wind. I stopped doing long distance interstate highway rides after that.