Patrick...good points...although the use I've tended to see people make of helmet mounted lights for the street has been as a secondary or auxiliary role to handlebar and seatpost or rear rack mounted tail lights. At least that's what I'm thinking the use of a double ended light would be. I've read that some people use helmet mounted lights to view the road or trail, but the strongest argument I've heard for them mounted atop helmets is for the 'be seen' purpose.

For a variety of reasons, aesthetics, aerodynamics, functionality(easy for it to snag onto stuff when mounted there.) I don't particularly like the idea of lights mounted on top of bike helmets. A possibly better idea for a helmet w/headlight-tail light would have the helmets' shell somehow shaped and contoured to provide a port for the light. But of course, since the primary purpose of a bike helmet is to provide shock absorption to the wearer's head in the event of a crash, the helmet mounted light being able to 'break-a-way' in the event of an impact to it would probably be essential so that the body of the light doesn't come between the helmet wearer's head and the surface being impacted, negating the shock absorption capability of the helmet.

There's been some discussion in this bikeforums category about the fairly recent kickstarter proposal for a bike helmet with integrated front light. People commenting here were wary of possibly poor ventilation...too low lumen lights. Manufacturers of fireman's helmets and motorcycle helmets have made some efforts to come up with lights on their helmets, but what I saw in web searches didn't seem to very desirable for use on a helmet used for biking.