IT IS RARE for one man's unorthodox opinions to become the conventional wisdom within that man's lifetime, but something close to that has happened to Edward Banfield, emeritus professor of government at Harvard University. Banfield himself would probably deny this, pointing instead to the amount of foolishness that still passes for public policy. And those who have changed their minds, however modestly, about those policies would probably deny that Banfield had anything to with it, not wishing to be charged with having learned anything from a self-confessed conservative. But consider: