I have had several friends who are practicing economists (Conservative and not necessarily in academia!) who say Krugman is legitimate on a certain narrow area of international trade. I am not an economists by academic training and I have never cared enough to educate myself on why on this subtopic of economics Krugman is not a crackpot Rat ideologue. So I take their word that Krugman is legitmate on a narrow sliver of disciplined intellectual thought.
Yeah, and when Krugman expresses an opinion about business siting decisions (the area for which he got his Nobel in Economics) I will give him due attention and defer to his expertise. However, his opinions on macro-economic policy seem to have less basis in fact than my own and his policy prescriptions based on them appear to be materially harmful to the interests of the United States and the world economy.
He is as useless in the area of macro-economic policy, despite being an economist, as I am in the area of analytic number theory, despite my being a professional mathematician. Of course, were I to get a swelled head and start making ill-founded assertions about the Riemann conjecture, it wouldn’t potentially have a harmful effect on our economy, society, and civilization, the way Krugman’s inflated self-opinion about his macro-economic expertise could.