I read here on BF the suggestion to carry a backup light, so I did for a while. My thinking is that my dynamo-powered headlight could fail, leaving me without a headlight. I try not to carry anything I'm unlikely to need. I don't carry spare spokes or spoke wrenches because while I have broken spokes on the road, it happens infrequently enough not to justify the trouble. And when it happens, I can deal with it. I can also deal with riding with no headlight if I have to. I've been riding with dynamo-powered lights for over three years now, and the failure rate of my systems is low enough to show me that I don't need a backup headlight. In three and a half years, I have had two failures: 1. one wire came out of the tail light. 2. While my bike was locked in public, a vandal tried to steal my headlight and failed. He bent the mounting bracket. The light still worked. After I got home, I bent the bracket back into the original shape. This really shouldn't count as a failure. So my headlight has failed zero times in three and a half years.

I cannot perceive the drag the hub creates. If I get home tired from the effort of my ride, I cannot rightfully claim that without the hub, I wouldn't be tired.