This will be an agonosing wrench for Hutton, who loved defence so much that in his old job (Work & Pensions Secretary) he would finish early, head over to the National Archives at Kew and go and research his various defence projects: books, plays etc. He felt passionately about the importance of our involvement in Afghanistan, the need to be vigilant on Iran and he felt in a minority there. As a hawk, in a government that skimps on defence, he fought like a tiger for resources to go to Helmand - ie the 2,000 more troops. But he (and the military) were overruled by No10.
Hutton would have wanted to stay and defend defence, if you like, as best he could. The preservation of a muscular Iranian strategy was particularly important to him and he was genuinely worried that defence would suffer cuts under the Tories. But he was also close to Purnell - he was Purnell's boss at DWP - and they think along the same pro-market, pro-reform lines. I have not spoken to Hutton, nor anyone close to him. But I do know him fairly well and I would suspect that he saw Purnell acting on his beliefs and thought: "I can't let him do that alone". And he'd have seen the weasel words of others - Tessa Jowell, David Miliband - and thought that he could not join them. He was always unlikely to be re- elected. But he could have spent his final year of his political career as a Defence Secretary, a job he loves, or a backbencher. He has chosen the latter, out of principle.
It's strange that Purnell has been called a "careerist" by John Prescott on his blog today. Choosing the backbenches over education, as Purnell did, is the act of principle rather than careerism. What David Miliband is doing is careerism: supporting a Prime Minister that he knows, in his heart, should go. I suspect Hutton judged he would not forgive himself had he not acted when Purnell acted: it is time for those who believe in a pro-reform Labour Party to be counted. And so far, today, I can count only two.