Talk of a currency war - in which countries try to depress their own exchange rates - has raised the prospect of increased exchange rate volatility. Is this a good thing or not? G20 finance ministers think not. "Disorderly movements in exchange rates have adverse implications for economic and financial stability" they said last week. But Nobel laureate Paul Krugman says otherwise: "The stuff that's now being called 'currency wars' is almost surely a net plus for the world economy." So, who’s right?