Just to add to this part of the discussion; I have Li-ion batteries that I've had for at least three years that are still very useable. A short while back I was testing one of my new lamps and was using my old batteries. I was amazed at how well the old batteries still worked.

I'm sure if you were to ride in the dark an hour everyday using the same battery, eventually the battery will lose much of it's original capacity. All the facts and numbers suggest this to be true but if you are only using them for occasional recreational use they should last a good number of years ( provided the cells are of decent quality ).

Another factor to consider with battery power lamps is how they function in cold weather. Li-ion batteries actually work well in cold weather. If there is a problem with diminished run time it is usually because the lamp head has built in low voltage protection. This protection works fine in moderate temperatures but when Li-ion cells get cold the internal resistance of the cell increases. This causes more of a voltage drop internally with-in the cell. The lamp circuitry can thus misinterpret this voltage drop to mean that the battery is losing voltage. If there was a way to readjust the circuitry for winter use this wouldn't be a problem but since there is no way to bypass the circuit, lamps that have the voltage sensing circuitry will at some point power down when they shouldn't. Some of my lamps have the circuitry and some don't. If you use a battery that has more capacity than you need then it usually isn't an issue either way.

If I was a die hard commuter and was using lights everyday I would certainly consider a dynamo if just for convenience sake. On the other hand if I commuted only occasionally by bike the battery power lights are fine. Like I said before if you need a dynamo for non-supported touring or endurance events a good dynamo set-up would certainly make a lot of sense.