Hrm, it's kind of a tough one.
Yes, your dog isn't likely to get it right now even if unvaccinated (the potential is always there though. It can be aersolized after all).
But, if we get enough people who stop vaccinating their dog for it, then you'll start to get more and more outbreaks and you won't be able to rely on the 'herd' immunity to protect your dog any more. People generally say to that "well I'll vaccinate them then" but that's kind of irresponsible IMO.
I don't really think they need it every 6 months though.
Additionally there's a school of thought (not sure how accurate it is, but just telling you what we learned in micro) that it's an opportunistic, commensal organism and that it can infect at any time if the dog is stressed enough. The thing I wonder then is, if that's true: why vaccinate at all? Because if your immune system is constantly fighting it off, it obviously knows what to do. The answer to that, I believe, is that it's not the immune system that holds it in check normally, but rather competition with other organisms in the dogs upper respiratory flora.
Gonna have to ask about that tomorrow.