As we advocate in our Agile Product Management overview, the more involved that a product manager is with the development team, the better. That involvement should be along the lines of a product owner who champions customer needs, the "why" of the product. When the involvement blurs into tasking, the "how" for a team, then there is a problem. Even with the best of intentions, this kind of utilization mindset tends to hide problems: defects, hand-offs, and unknowns. Interleaving scope and process tends toward locking scope, schedule, and quality. That's a recipe for failure.

When starting out with scrum, it can be a huge help to have someone in the role who has seen scrum working before. Better yet, has seen many examples of it working. For this reason, scrum masters are often hired as consultants, rather than as full-time employees.