That's the point. I don't have to buy one for each bike. I buy one light system that I can fit to all of my bikes. If I want to ride my commuter bike, I plug in the batteries and take off. If it snows and I need to ride my winter mountain bike, I move the lights over to that bike and off I go. If I happen to want to cruise after dark on the cruiser, I swap the lights and go cruise. If I want to go mountain biking, same thing. If my wife want to come with me, I have enough lights that I can put them on her bike and on my bike and we can go for a night ride. That's maximum flexibility at a reasonable cost.

Compare that to doing the same with dynamos. I have 26" wheels without disc, 26" wheels with disc, 700C wheels for a race bike, 700C wheels for touring and commuting. And my wife has a 26" wheel mountain bike and a 24" wheel Terry. So to go the dynamo route, I would need, at a minimum, a 26" disc, a 26" wheel rim, a 700C rim, a 24" wheel rim and another 26" wheel rim. I'd need harnesses for my 7 bikes and her 2. Maybe I could move the headlamps around but that's hardly "convenient" enough to just hop on the bike and ride, is it? And cost wise that's going to run to around $1000 worth of equipment.

I could equip just one bike with a dynamo system and use it all the time but that's not why I own 7 bikes. I enjoy switching them up and using a different bike so that the commute (most of my riding is commuting) isn't quite as boring.