I honestly think it depends on the quote and the situation. If you're writing an article, then I've read a few which began with a quote that really got my attention. The quote seemed to encapsulate the whole article that followed. For a speech, I think it can work the same way - and sometimes be more effective. I think a good speaker can deliver a quote in a way that makes it come alive. After all, many famous quotes were spoken aloud in the first place!