You can learn a lot by coding for 12+ hours a day for 12 weeks in an environment with a) more experienced people readily available, and b) other students working on the same or similar problems. I had been teaching myself for about two years prior to attending, and having other people to talk about coding with (SO does not count in the sense I'm referring to) was a huge help. The staff were also pretty legit.
I don't think that people will generally choose a bootcamp grad over someone with multiple years of coding experience, but that is primarily because they're not competing for the same jobs. I think as a bootcamp alum you're competing initially against recent CS grads.