A blog post by Jonathan Portes, the director of the National Institute of Economic and Social Research, in which he lambasts the European Commission for dealing with Fitch Ratings, has been spreading far and wide, and for good reason. It's worth reading the full post, but here's the tastiest passage:

These agencies have repeatedly been proved wrong; they have flawed and frequently conflicted business models; and their ratings have no predictive power.  All this is well established. Moreover, when it comes to assessing sovereign debt "credit risk" they - and I mean this quite literally - do not know what they are talking about. By that, I mean they quite simply don't understand what they themselves are saying.

Paul Krugman agrees:

We saw very dramatically what the rating agencies are worth when S&P downgraded America — nothing. Bond yields actually fell.

The point is that while maybe, maybe, S&P or Moody’s or Fitch know something about corporate debt, they know less than any competent macroeconomist about sovereign debt.

A good way of sorting the economists from the political commentators appears to be whether they have consistent views on the ratings agencies. Compare the attitude to the two times Britain has been put on negative outlook – once under Osborne, once under Darling – and you will find a lot of contrasting views. Either many commentators had a radical conversion to or against the expertise of the agencies, or there are a lot of charlatans on both sides of the political divide who have no strong views on credit ratings agencies beyond "they are a useful stick to hit my opponents with".