The idea behind the op ed page was to promote diversity of opinion by letting the opinions of people who were not professional journalists as such have an avenue for expression. Of course we see at the New York Times, for example, paid staff who are professional journalists also writing op ed pieces.
Op ed pages also provide a palce where scholars and academics can express themselves and influence the reading public. A recent study of the op ed pages at The New York Times, The Wall Street Journal, and the The Star-Ledger (the major New Jersey paper) has revealed the op ed pieces accepted for publication from academics AGREE WITH THE EDITORIAL STANCES OF THE PAPERS ON THE ISSUES WRITTEN ABOUT 90 to 95% of the time. That is not diversity, it is propaganda.
This shows that to really get a different take on the world than that dished out by the bourgeois media you should be reading Political Affairs, The People's Weekly World and other left publications. Despite Fox News, The New York Times is NOT a left publication!
(this news is from the NYT 6/23/08 "Media Talk" in the Business Section).