“Teams need to understand who does what and how the various work fits together. This becomes even more important as companies grow.” - Brian de Haaff. 

The product owner role is really critical in an organization as it helps in translating the requirements to final PRODUCTS. As we have been discussing so FAR, it is a full-time JOB which requires constant collaboration with the clients, pulling up the feedbacks, working on the backlogs, helping the teams, etc. But if someone with an existing job title tries to fill in, he or she will not be able to JUSTIFY the role and in turn, the product and the team suffers. 

There might be delays in the feedback loops, the vision and roadmap might not get clear to the team, even the backlog suffers as it requires continuous attention. It is not advisable to take someone with already a title to play this role. The organizations need to focus on quality and time to market to stay up-to-date, hence, a person with dual role might not be able to substantiate such expectation from the role. Also, it will impede his or her efficiency in their former role. Though, in very small organizations where the team size is small and the teams are directly interacting with the clients, in those CASES, this dual role can work.