Paul Krugman: Economist Or Activist?

I hadn't realised quite how single-minded an activist Krugman had become, so coming across one of his columns recently was an unpleasant surprise. In Easy Useless Economics he argues that the economy's problems, and especially unemployment, are all about demand shortfall and that anyone talking about structural problems is lazy, deluded, and/or a hater of the unemployed.
As Krugman puts it,

So what’s with the obsessive push to declare our problems “structural”? And, yes, I mean obsessive. Economists have been debating this issue for several years, and the structuralistas won’t take no for an answer, no matter how much contrary evidence is presented. 
The answer, I’d suggest, lies in the way claims that our problems are deep and structural offer an excuse for not acting, for doing nothing to alleviate the plight of the unemployed.

Really? Can't one have more than one explanation in something as complicated as an economy? Isn't real social science concerned with working out how multiple causal mechanisms interact, not which one is correct? Do people who suggest other - heretical - explanations deserve demonising and insulting? Is that how progress is made in science?
Krugman is in danger of becoming a deformed creature of polemic. No more the Krugman we used to love, the hero of the intellect who won a Nobel Prize, but some kind of screeching Keynesian harpy.