What is the issue with using the helmet light? Is it too bright, is there a reflection off a clear window over the map, or is it just inconvenient to aim it?

When using a monochromatic light source, like a red LED, you can't distinguish colors which can make reading a map difficult or impossible. Even though red can help preserve dark adapation, a very dim white light may be a more optimal solution if needing to read a map while preserving night vision. Preserving dark adaption is not really an issue if you're riding with lights, though....