The convenience of a hub dynamo and a LED dynamo light is great; you never have to worry about batteries or whether you need a light or not later that day since you always have a light source handy. Just jump on the bike an go.

My dynamo hubs and lights have also been the most reliable light systems I have ever owned, they just works day in and day out for years without trouble. I think some of its reliability is because the light source is bolted to the bike, and that it is very light since it doesn't contain heavy batteries and therefore doesn't rattle itself to death like some battery powered lights I have owned. Since the light is permanently attached, you don't drop it on the concrete floor either; putting on and taking off lights twice a day means that most people will drop them one time or another.
I also like that when visibility drops in the daytime because of rain or snow, I just turn on the light.

Not having to depend on batteries is a big deal for me, I simply got tired of researching batteries and battery technology; Nimh, Li-ion, Li-Po, NiCd, LSD, protected or unprotected, is this battery pack charged in parallel or serial, which battery has memory effect, should you top off this battery type every day, or does it harm it, delta trickle charging is a must have feature except for these new Zinc batteries, does this battery explode when overcharged, etc etc.

Many rechargeable batteries have unpredictable shorter run times when it is very cold, but also have so high a self discharge that spare batteries becomes a logistic burden.
Many new high power batteries can be quite dangerous if damaged or if they short circuit, eg. one guy who had bought a Magicshine light, had to throw it out the train window when it suddenly caught fire for no apparent reason. So you really have to babysit them, taking care that they don't get damaged, and use quality chargers etc.

For my usage pattern, batteries are too much trouble. Just the fact that I don't have to carry bulging lights in my pockets when stopping for an errand makes me prefer dynamo lights.

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Regards