Ability to disengage a bottle generator is one convenience. The other is the ease with which you can install and uninstall them. It's easier than building a wheel and cheaper than buying a wheel.

My Raleigh Twenty is my project bike. It has a drum brake front hub. Changing that to a dynamo hub would be a big investment, especially a dynamo-and-brake hub. In this light, a bottle dynamo makes more sense for this bike. I can feel the drag, but I won't be running the dynamo on long rides on this bike.

I plan to give one of these bottle dynamos away to my young friend Al who was my housemate in NJ. He doesn't have much cash, and since I gave him that Super Course, he has switched from riding the bus to riding the bike a lot. He would like lights on it. I can give him some lights with filament bulbs, and perhaps he can buy himself some LED lights. Giving him the bottle dynamo will save him a big pile of money, and running either type of light -- filament or LED -- will be a lot convenienter than running battery lights.