Krugman was very influential in the development of a theory of international trade not involving perfect competition. Whatever you think about economics in general, it seems to be a pretty clear step towards more closely modeling reality.
In other words, Krugman's contribution is a lot of math that has never been shown to work. When you add math to a theory in a way that does not improve people's understandings of the issue, you have not added to the world's store of knowledge.
I've read a bit too much criticism of economy as a science using the same argument you see from creationists against palaeontology. (That is, assumptions that all researchers are idiots and/or in a conspiracy against the creationists/left wing extremists.)