Back when Metallica played with the San Francisco Symphony Orchestra, a comedian on TV made a very funny observation: if a bunch of metalheads could play without a scoresheet, why did the orchestra need one?
The other logical question that he did not raise, but people often ask is, if the music is already written down, why does an orchestra need a conductor? Can't a group of 100 musicians just pull out the scoresheet for Beethoven's Ninth and start hammering away?
The Aug 21st issue of The New Yorker has a superb article titled "Measure For Measure" that examines the job of a symphony conductor. The article isn't online, but "Hard Drive" has a 27-minute video featuring clips of great conductors and the article's writer (Justin Davidson) talking about the conductor's work.
When people say they prefer Beethoven as conducted by a Mehta or a Karajan or a Solti, just what do they mean?
If you cannot access older issues of the magazine, watch the video and you will know just why an orchestra needs a conductor. (Link to New Yorker's Hard Drive)