Most people that use bamboo do it because the investment is much lower than a metal frame. Calfee got into it so that people that had bamboo and not much equipment could make bike frames.
People that haven't built a frame and want to build a frame are always looking for shortcuts, and bamboo is a pretty good shortcut. If you look through the archives of this forum you will find people that want to use all sorts of materials to build a frame. Like PVC, which is a horrible idea