In my view, a project manager assigned to an Agile project can fill the role of Scrum Master/Product Owner/Team member according to his/her inclination, experience, background, skills, etc.
But assigned to a project where these roles have already been filled? I believe the value add is in coordination (eg. virtual teams), internal/external communications, customer relationship (supporting PO), contract management, risk management, quality management...
Comments, please?