The cyclist is never 'caught out' after dark with a generator. A generator costs nothing to run. A generator can easily drive a good bright 3W halogen bulb giving a very useful front beam. A generator system is lighter in weight than a battery system capable of similar power output for more than a couple of hours. Some Drawbacks Noticeable drag is the most significant problem.
Many generators are also quite noisy in action.
When a cyclist stops, the lights go out. Similarly, at very slow speeds, eg, climbing at around 5mph, the lights go dim. This is particularly noticeable with bottom-bracket types. And at very high speeds too much voltage can be generated causing the bulb to blow. Again bottom-bracket types are more likely to cause problems, but it is simple and cheap to fit a voltage regulator to any generator system to overcome this.