I agree with the principal of backup lights, but I'm curious. Have you ever had to bring in a battery light because a dynamo light has failed? I haven't, not once. I have had plenty of battery lights drain or fall off the mounts and fail in other ways that having backup would have been a good thing.

Once I noticed my dynamo tail light wasn't working. One of the wires had fallen out of the light. That's the only failure I have had.