I wouldn't normally link to Tim Leunig, but

Here he is on the White Paper, and for once I think he is right. Core/Margin will make little difference to anyone if the margin is 5%

Update 14 July 2011. I heartily regret giving Leunig this link becasue he was, as ever, wrong. I've since blogged on core/margin here, here,  here, and here, so pretty clearly I think core/margin is a big deal.