When a critic votes with a vast majority, I think one reason is that some films are obviously good or bad (in the eyes of most people). But when one lonely critic stands apart from the mob, there may be a message to be learned, and that may be the critic you should make a point of reading, assuming he or she has been interesting in the past. There may be a special expertise or sensitivity coming into view, or a film may have been made with such specialized intent that its qualities are invisible to the majority. Or, sometimes, it may be the auteur theory at work, and the critic may be so invested in the work of that director that he or she sees things that reach specifically to his wave length.
Why do critics run in packs? One reason Ebert doesn't really get into here is that so many Hollywood films are specifically tailored to reach the broadest possible audience and are so bland as a result they fail. If you've seen Made of Honor, for example, can you really imagine having a serious disagreement with someone about it? Or preferring it to an independent film like Ira & Abby, which I just saw on DVD? (Roger Ebert's Journal)