Sure, that Smith's husband chooses to watch porn films in the privacy of his own home is a personal matter. But the moment when the taxpayer is expected to pick up the tab is the moment it becomes a public matter. It remains a public matter even after Smith has apologised and paid the money back, as taxpayers need to know that this kind of thing won't be allowed to happen again. There cannot be a presumption that politicians can claim for almost anything under the sun - "mistakenly" or not - and only rectify the situation once they're pulled up on it. Quite simply, the system needs to be made more transparent and less open to claims of the extraordinary kind.
Thing is, Downing Street has done its best to distance itself from all the controversies of the past few weeks. As Paul Waugh has noted, they've conjured up a variety of rhetorical sleights which add up to the same thing: "Nothing to do with us, guv". Yes, there's going to be an investigation into MPs' expenses and allowances. That's a welcome first step. But Brown's evasions suggest that the political class doesn't regard the matter with any urgency.
P.S. Cameron's take on the situation is better, as he urges that "We need complete transparency over expenses". But the way his "new politics" talk of last year didn't come to much doesn't fill me with a great deal of hope.