Products are developed through a pipelines of tribes, each one seeking maximum say in the development process. Marketing, designers, developers and sales have their own culture code that do not allow them to develop complete or holistic product development view. As a PM its your responsibility to make them listen to each other in a constructive way and to understand the whole product. This means you have to be polymath or create a perception that your are. All in all this requires amazing amount of persuasive skills on PM's part.