When the goal is to provide excellent value to your customers through your product, then the importance of having the Product Management folks involved in the overall Lean effort cannot be overstated.
Too often organizations are broken up into functional silos that are disjointed, rivalistic, and unable to effectively communicate with eachother. This is the opposite of Lean. Instead, businesses should be organized into Value Streams that are focused on their customers. Managing by Value Streams allows for the quickest flow of information and materials possible with each Value Stream establishing its own performance metrics based on that Value Stream's customer's definition of value.