Paul Krugman:
I don’t have time right now to track down all the examples, but if you look at how many freshwater macroeconomists have responded to Keynesian arguments in this crisis, you find over and over again that they resort to assertions of privilege — basically, I am a famous macroeconomic expert and you aren’t — rather than really addressing the issues. [. . .]
[T]his is never an appropriate way to argue — least of all at a time like this, when events have strongly suggested that a lot of work in economics these past few decades, very much including the work on which these guys’ reputations are based, was on the wrong track.
Do I do this myself? Probably on occasion, when I don’t catch myself. But I try not to. I would say that commenters who begin with “I can’t believe that a Nobel prize winner doesn’t understand that …” might want to think a bit harder; mostly, though not always, I have actually thought whatever you’re saying through, and the obvious fallacy you think you’ve found, isn’t. But “Me big famous economist, you nobody” is not a valid argument.
Moral of the story - think for yourself. Saying "big famous economist sez" is persuasive on some level, but not dispositive.
UPDATE - some substantive argument on this point here and here.