It is commonly agreed that team managers should not be scrum masters, but I am struggling to see why. For context, I am an Application Development Manager with 4 devs in a Scrum Team. I come from a Scrum Master background, and have introduced scrum to the organisation. I have built the team from scratch and made it clear that everything I do is to facilitate the team, and that they make the decisions. As a team we are very open - they even silenced me at stand-ups for a while to eliminate the 'reporting' feel we were starting to get. A lack of openness is generally the biggest argument against the manager as scrum master, but handled well, is easily overcome with the right culture.
I've been warned by experienced scrum coaches that this is a dangerous situation, and there are risks 'if things go badly'. The way I see it the 2 positions do not conflict, in both roles I have the same aim for the team and individuals. Scrum resolves conflicts within the team, which could traditionally be a managers role. The self-managing nature of sprints takes away the allocation of work a manager would traditionally do.
All I really see left to pick up as a dev manager is making sure individuals needs are met, career objectives, workplace etc. I have a weekly catch up with each team member to raise any issues, and handle any admin tasks. A lot of this relates directly to the team, or my role as scrum master anyway.
I understand in large organisations how this could be unmanageable and a separate role but for a small organisation we could certainly not justify another Scrum Master or Development Manager.
Please enlighten me as to the pitfalls of Development Managers as Scrum Masters, excluding the points I raised above and have already overcome.