Engineering should be involved when designing the DID. The engineers will be able to give a more educated feasibility rating. Also, even though details are not needed as much in the DID as other aspects of the project it is still important to be able to provide enough information that it could be understood. A customer and a marketer might have a great idea, but engineers will be able to make sure the project stays within reason. The customer also needs to be involved in the Design input document in that they need to make sure the product is following their desires and customer needs. A failure to meet customer needs is, in most cases, a failed project. You may stumble upon a new product, but most likely will result in having a project that is not desired by anyone. This results in a huge revenue lost for the company as now the projects cost has had zero return.