It's easy enough to find bright lights for $30, but that doesn't make them good bike lights. At that price point, a bike light if far more likely to be junk than not. The CCT is likely to be way too high, and the electronics are unlikely to be potted. At that price point, many lights will have poor thermal management (a very big deal for bright LEDs). Those lights aren't going to provide a beam nearly as good, or as well suited for cycling, as a quality generator light.

Fair point. Some lights are easier to switch between bikes than others. But often, two different bikes see different typical speeds or riding environments, in which case a single light won't be well suited for both bikes. It isn't as easy as selecting a brighter mode. Different riding speeds and environments require (optimally) different beam shapes, which is typically beyond the capability of a single light.

I have lots of time and money invested in my battery powered lights. I love them. I have over $200 invested in just the rechargeable cells that I use regularly. Still, I can think of very few good reasons to choose a battery powered light of a generator powered light on an on-road bike.

One would be the ease and quickness with which some battery powered lights can be removed from the bike. (For me, this is a positive, but it's a negative if one doesn't actually remove the lights when not riding the bike.) A "permanently" mounted light is not as theft resistant as a light that the owner removes from the bike when not in use, if the owner actually removes it.

Another reason is that a battery powered light, particularly a good flashlight (or two) mounted to the bar is quite useful when off the bike.

Neither reason is compelling.