Dedicated Scrum Masters

The SMs I've worked with who've had the most positive effect on a team (performance, safety, empowerment, etc) were dedicated to the SM role. Everyone I have spoken to who has worked with a competent dedicated Scrum Master agrees. 

Every 'Developer as Scrum Master' (DSM) I've worked with has been far less effective. They can keep things ticking over but in my experience the DSMs put much more into the Dev role.

The real test is when a crisis happens: 

• Do they go into SM role? Do they focus on facilitating the urgent re-planning and discussions? Do they work to ensure safety so that the Developer's don't work silly hours?
• Or do they go into Developer role? Are they active in tech discussions and drawing on the whiteboard? Do they put in extra hours coding?

The DSMs I know would go completely into Developer role in a crisis and drop most Scrum Master duties in the process!

But it is more than this. DSMs are more likely to look at Stack Overflow for a coding issue rather than read a Scrum.org blog post about a team issue. They do code show-and-tells more often than run workshops teaching Agile supporting practices. They more often step in to the role of the expert telling the team rather than the active listener asking powerful questions.

The Head-of-department Scrum Masters I've know have had some success on tactical issues (impediment identification, not over-~committing~forecasting in Planning, etc) but have not really been really effective as regards kaizen and taking the team to the next level.