According to my friend Jeff Sutherland, co-creator of Scrum, Scrum Master is a full time role and here is what he stated in his Scrum Handbook -
Since Scrum makes visible many impediments and threats to the team’s and Product Owner’s effectiveness, it is important to have an engaged ScrumMaster working energetically to help resolve those issues... Scrum teams should have a dedicated full-time ScrumMaster, although a smaller team might have a team member play this role.
Did you get his point? Read this again and focus on the highlighted text above.
It is a full time role and job both, probably easy to digest for large teams but most would disagree that this is a full time role for an average size scrum team. Going by Jeff's statement above - in a smaller team (6-8 members) a team member may be playing the role of scrum master. Mind it, even in this case, it is a full time role as this member might be involved in coding-testing activities but he would still be spending good time doing servant leader activities as well - like removing impediments, continuous improvement, ensuring scrum is followed, and so on. In a larger team this role gets to spend more time doing all of that and lot more.
- scrum masters supporting more than one scrum teams - team or project compromised? Most likely yes!
- scrum masters involved in tasks not related to scrum or scrum team - compromised? Yes
So think again if you or any of your teams practice part-time scrum master role.