UNTIL RALPH NADER spoke out the other day, it had not occured to us that we need an organization to represent us against "the rising crescendo of fraud and fast-buck artistry" in the world of sports. But there he was at a press conference, Mr. Consumerism himself, announcing the formation of FANS - Fight to Advance the Nation's Sports. FANS, he said, will represent the interest of sports fans everywhere "before the leagues and players' associations, individual owners, the broadcast media, Congress and other federal bodies, state and local governments, the courts and other appropriate forums."

After hearing that mouthful, we began to wonder. Being a sports fan, after all, is a little different from being a food buyer - or a taxpayer. You don't have to be one if you don't want to be. And if you are one, your interest may be quite different from that of your neighbor. Take, for example, a baseball trade that Mr. Nader deplored and indicated FANS would have attacked if it had been in existence: the trade of Tom Seaver from the New York Mets to the Cincinnati Reds. Fans of the Mets, no doubt, though that was a terrible trade that should never have been made. But fans of the Reds thought it was pretty great. FANS, we suppose, would get around that problem by describing itself as representing the interests of "real" sports fans, as distinct from parochial fans.

The point is that the idea that organizations can speak for groups of people is getting out of hand. The National Rifle Association doesn't speak for all of its members; neither does the American Legion or Common Center or (the good Lord knows) the Democratic Party. But we suspect a national convention of FANS would make the quadrennial political conventions look like tea parties, and not the historic Boston variation either. Can you imagine the debate over whether it is in the best "consumer" interest of sports fans for Howard Cosell to continue broadcasting - or whatever it is that he does - on Monday nights? Or over whether to applaud or deplore the length of the professional basketball season?

There are a good many things wrong with the world of sports. It is full of arrogance, greed, cheating, brutality and cold hot dogs. These ought to be attacked - and are each day - by sports writers and broadcasters. They are even attacked sometimes by fans who join together to back one particular team. But can you imagine the fans of the Redskins and the Cowboys coexisting peacefully in Mr. Nadar's new group?Besides, the founders of FANS are against the moving of sports franchises from one day city to another. And you know from that what side they will be on if a baseball team in San Diego or Oakland or someplace else decides to move to Washington.