The sole purpose of the question was to embarrass the Prime Minister and ensure that any headlines in today's newspapers focused on this issue rather than what was otherwise a strong interview by the PM with a good policy announcement on the banks attached to it. As Alastair Campbell put it on his blog:
I know it will give him the passing satisfaction of pats on the back from journos whose backs he pats when they come on to do their 'excellent, as ever' reviews of the papers. But it was low stuff. I'm sure Andrew would agree that everyone has certain areas of their life that they'd prefer not to be asked about live on TV.