"being an economist involved in the policy process is harder than being an economist in academia. Academics can easily say "I don't know" and move on to another question. Policy advisers are often required to take a position on the key issues of the day, sometimes in the absence of reliable evidence."
This to me is precisely why economic advisors have no business being policy advisors. A true economic advisors, should in theory at least, provide any data available - describe what that data means based on various realisitic assumptions, and provide any alternatives that the data might imply (based on other assumptions). It is role of SOMEBODY ELSE to take that snapshot and add in others' snapshots (advisors from other fields) to determine what a specific policy change might have an effect on. My question to Mankiw is, when did economists cease being economic advisors? When did they cease recognizing that their worldview has limitations in the real world? When did they start trying to be the be/all end/all of decisionmaking? Such shifts seem problematic to me (as they apparently do to a number of commenters on his blog).