I think the greatest lure of bamboo is tradition. Fly fishers as a group are more traditional than spin fishers and it stands to reason that some of these fly fishers would favor bamboo. Truth be told, graphite is an overall superior material for fly rod construction than bamboo, but that does not mean that in some conditions bamboo can excel.
Have you ever heard of a Morgan automobile? They are still being made and the unique aspect of the Morgan is that the frame is made of wood - ash wood, just like baseball bats. Why would anyone want a car with a wooden frame? For much the same reason as some fly casters want a bamboo rod. Some people enjoy the fact that they have something that is unique and special. For these folks it is more about the journey than the destination.
Regards,
Silver
"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy