Sir Dennis Robertson, quoted in The Calculus of Consent (available online), offers the best description of the role of the economist I have read yet:

There exists in every human breast an inevitable state of tension between the aggressive and acquisitive instincts and the instincts of benevolence and self-sacrifice. It is for the preacher, lay or clerical, to inculcate the ultimate duty of subordinating the former to the latter. It is the humbler, and often the invidious, role of the economist to help, so far as he can, in reducing the preacher’s task to manageable dimensions. It is his function to emit a warning bark if he sees courses of action being advocated or pursued which will increase unnecessarily the inevitable tension between self-interest and public duty; and to wag his tail in approval of courses of action which will tend to keep the tension low and tolerable.