It is certainly the case that Krugman's new theories of international trade have had an enormous impact on the field of international economics. The Royal Swedish Academy have provided an accessible summary (PDF file), with the briefest of footnotes to his academic contribution under 'Other endeavours'.
In addition to his scientific research, Paul Krugman is highly appreciated by his students as a pedagogical lecturer and author of textbooks. In wider circles, he is better known as a lively blogger and spirited columnist in the New York Times.
This emphasises that, like the John Bates Clark Medal, the prize is awarded on academic grounds. But what is most valuable about Krugman has been his equal insistence on economics as a public science, and of the importance of expert contributions to policy scrutiny and public discussion. Part of Krugman's genius has been his ability to write for and reach very different audiences - as his official web-page demonstrates - long before his New York Times columns made him among the most prominent economic and political commentators. (And his supplementary blog to his NYT columns is an exemplary case of both public communication and perhaps also all-too-rare commentarian accountability).
My first job after leaving university was in book publishing, working for Macmillan for whom I commissioned the academic economics and politics lists. One slight downside was that my birthday always seemed to coincide with the Royal Economic Society's annual conference, which was the key UK gathering of the economics profession. And it did not help that, by contrast to the Political Studies Association conference, there seemed to be an ever decreasing probability of a non-specialist walking into a session and understanding something of what was going on.
There were certainly exceptions - and energetic attempts to communicate a few eye-catching findings to the newspapers - but the legacy of Keynes seemed at great risk of being lost in a flurry of econometric models and stochastic equations.
The causes of the narrowing of this public science may be many and varied. They certainly include academic funding regimes which focus on the quantity of articles published and places little value on public contributions. A long and unresolvable debate could be held about how to judge what balance should be struck between the intrinsic value of academic work and its public communication. This might not be a fair point in medieval history, botany or many other disciplines. But in the social sciences, if there is barely any attempt whatsoever to communicate outside the academy, it is difficult to understand quite what the point is.
So those first rate academics who insist on breaking the mould provide an important and undervalued public good.
Of course, personally, I am attracted to Krugman's writing by his liberal (and social democratic) values, as well as his punchy and pugnacious style: he has made important contributions to the debate about inequality in the global age, showing how increased inequality in the United States has been a product of political and governmental choices, and not simply technological change.
But, whether you agree with him politically or not, Paul Krugman stands very much in the tradition of Keynes - and indeed also of Hayek, Friedman, Ricardo and Adam Smith - in understanding what economics, as a public science, should be for.