Men and women go into politics as a career, and in order to transform society according to their ideals and beliefs. These are complementary motives and not either/or choices, but surely, at least for "left of centre" politicians, the second should predominate. Even if David Milliband is not, after all, to be leader of his party, he still has ample opportunity to contribute towards the creation of what is left of the socialist ideal. If he decides not to do so then he will seem like the playground footballer who refuses to play unless he can be captain.
The most urgent task of the present parliament, for all parties, not just the coalition, is to restore respect for politicians and confidence in the political process. If David Milliband decides to quit, that will add to public cynicism rather than restore respect. Service under a younger sibling may mean that a certain amount of pride has to be swallowed. I can't think of any exact parallels, but there are several examples of politicians continuing to serve under former juniors. Alec Douglas Home continued as foreign secretary under his former junior Ted Heath, as does William Hague under Cameron.
If David Milliband decides not to continue in front line politics he will confirm the suspicion in many people's minds that politicians are "only in it for what they can get." I hope that he will decide to stay and put what remain of his principles first, however misguided I believe them to be.
A post script: a young man in our Co-op appears to be reasonably interested in politics, and, I believe, represents the staff on a regional committee. When I asked him yesterday how he was, he replied that he was worried about where Ed Balls might lead the Labour Party. With a recognition factor like that Ed Milliband needs all the help he can get.