I am very happy that more and more discussions are held what the role, responsibilities and activities of a Scrum Master is. The Scrum Master Manifesto is such an initiative. I agree with the Manifesto authors that Scrum Master role is a fulltime job when working with a new Scrum team and team members new to the Agile and Scrum way of working. But I am convinced that a Scrum Master can support multiple experienced Scrum teams.
I worked with quite a few Scrum Master, Product Owners, and Scrum teams. I am convinced that it is more productive and effective that the Scrum Master works fulltime as coach and is not working on stories. It was very often a huge stress for Scrum Master being team member to prioritize their Scrum Master responsibilities against the commitment of the sprint goals. So I strongly recommend to have fulltime Scrum Masters.
More important is that you have real Scrum Masters, and not only Scrum administrators just organizing meetings and managing the backlogs, the Scrum board and the impediments list. I challenge you to read the Scrum Master Manifesto and check yourself. Do you live the 12 Scrum master pocket principles? Do you perform during each sprint the top 10 things a Scrum Master usually forgets to focus on?
If you fulfill all the criteria of the manifesto please send me an email. I would like to work with you. If not the improvement measures should be clear.
I hope that the discussion what a successful Scrum master do stay active. It is a huge gain for the Agile community that people realize that a Scrum master is a coach, a line manager, and change catalyst.