I'm interested in it for longer touring when there's no way of charging my Hero via mains. I'm going on a wild camping tour of the highlands of Scotland later this year and we'll be out in the wilds for about ten days. I'd like to be able to charge my phone while riding, not only so I can read (got kindle on it), but also listen to music at the end of the day. GPS would be good, also. My battery last about six hours when I use GPS. I figure that I'd happily trade off some resistance for a charged battery.
Ideally, I'd go with a hum dynamo, but I have a 700c tourer that I'd probably also want to use it on when away in Europe, and two dyno-hub wheels for both bikes just make it too expensive.