For those who have read my prose elsewhere, I'm a firm believer in generators in general, and more so in dynohubs and even more so in the Schmidt dynohub. However, as others have said, there is no need for one in most touring situations. That's the key point; still it's nice to have.
Machka has already covered a few untypical touring situations where the dynohub is useful. While touring, I have used mine a couple of times on overcast and rainy days. With the sun in my face however, a powerful flashing blinker is more useful. But there are many cycling situations where I enjoy mine: commuting, nice evening rides, whether in town or out of town.
There are a few advantages of the generator system :
– The light is never out. I think that's an under-estimated factor. Even when justriding around town on a wonderful night, it's nice to know that I don't have to plan my ride with a given time limit. I can go as far as I feel, ride as long as I'm not tired, yet know that I will be able to ride home with plenty of light.
– No dangling wires around. Small battery headlights have their batteries in the casing. But larger ones – the ones that are more powerful than a Schmidt dynohub – typically use an external battery pack which either replaces a water bottle or hangs on the top tube. Even in the dark, I hate dangling wires around a bike.
Finally, for self-contained touring without a dynohub, I would recommend against a headlight with a rechargeable battery pack. Apart from the bulkiness of the load and the fact it typically uses the same handlebar real-estate already consumed by the handlebar bag, rechargeable battery packs tend to self-discharge anyway. So you charge it at home and have 2-3 hours autonomy... except when you need it 1-2 weeks after, there's hardly any juice left.
The current crop of LED be-seen headlights are useful enough to be seen, to serve around camp... and to light the road (don't ride too fast) in an emergency. At home, they can be used with AA rechargeables, but on the road they can be used with disposable lights.