AFAIK, they don't. Many longer randonneuring events in the U.S. do, 300km and up. The inspector at PBP gave me no end of trouble because I had a spare dyno light on the bike as a spare, but making it work was going to be an exercise in re-wiring. Not sure what the French word for "spare" is, "plus?" I just got new lights and being superstitious.

I have a spare battery headlight and taillight on my dyno-equipped road bike. I use the battery headlight to augment the dyno light sometimes, do the same with my helmet light