I'd say there are three main reason, in decreasing order of legitimacy: 1) he uses his academic credentials to write a more-or-less partisan column 2) a lot of the time he can't just disagree with someone, he has to call them an idiot and corrupt as well 3) he's a keynesian and hard money supporters are loud on the internet

When he sticks to economics (his blog has a higher percentage of economic material compared to his column) he is definitely worth reading and shouldn't be dismissed with ad hominem attacks.