I don't believe you don't know the answer to this. This thread started as a roadie picking out a Brompton, then somehow we got sidetracked into lighting. So I surmised that if he cared at all about lights (which he never mentioned), he would probably appreciate lightweight USB pieces that are only on the bike when he needs them. And there in lies your answer: Weight. I love my Shimano dynohub with B&M Eye Tec on my Rivendell, but I wouldn't want to lug that stuff around on a folding bike I'll be carrying a lot. For that application, USB is better. You're welcome.