The most important role in the Project is the Stakeholder! They're the ones who want it, and they're the reason we're doing the Project anyway! In my experience, failure to not involve Stakeholders, i.e. the customer for the Project, leads to the delivery of Products and Features that simply don't meet the requirements of the End User. In 9 out of 10 Projects, making decisions internally without external consultation will cause you to either deliver the wrong solution, or just part of it...badness!