See Pages 171-172 of Tracy's first book. It really articulates her take quite well. Referring to co-sleeping, she says "... If ... these practises work for you, for your baby, for your lifestyle, then by all means stick with it. The trouble is, the people who call upon me for help [basically started down this path, and couldn't see it through ...]"
What she seems against (and encourages pre-empting) is parents starting with co-sleeping (without thinking it through)--then suddenly deciding they don't like it after a few months. Sadly, this happens a lot. And, it forces sudden, dramatic changes on little babies that are unfair and avoidable.
So, if at 6 months, everyone is happy with co-sleeping, don't change it. I think you can still definately benefit from the BW insights.