Jeff at Midcourse Corrections thinks big-name speakers who are hired just for their big names actually are beyond the "becoming" phase when it comes to irrelevance, and that they may just make your meeting -- and your organization -- irrelevant too.
Read his post for all the reasons he thinks celebrity speakers who don't customize their messages should go the way of the dodo bird. I agree with him, for the most part, though, as I noted in a comment on his post, I'd hate to see a conference get so insular with its content that ideas that aren't directly related can't get in. While I can only speak for myself, of course, I've found that some of the best ideas come from things that have nothing to do with the topic at hand, but spark me to think differently about that topic. I can't tell you how often something from meeting planning has changed the way I look at journalism, for example.
Targeted content should of course be the main course of any conference mental buffet, but I wouldn't rule out throwing in some unrelated, yet thought-provoking, appetizers as well.