To be really honest here, that is a useless parameter when used with custom taxonomies, as custom taxonomies are already registed to a custom post type when they are registered. Using the taxonomies parameter, you are re-registering a custom taxonomy to the custom post type
As stated by the codex, the taxonomies parameter is used to register build in taxonomies to custom post types, as build in taxonomies are registered to the build in post type post.
Just to close of, if you register a taxonomy to a post type, set the taxonomies parameter and use register_taxonomy_for_object_type(), you are doing the same job three times 🙂