We rarely recommend memorization of a fixed text for speakers, but almost every speaker will, at some point, need to quote another writer, pundit or historic figure. And when you do, changing your tone and varying your vocal qualities to fit that special text will help you lure in your listeners and emphasize your points.
Need a mentor? Listen to this recording of high school student Shawntay Henry reciting from memory the poem "Frederick Douglass, by Robert E. Hayden. Henry just won the 2008 Poetry Out Loud competition and a $20,000 scholarship for this rendition, which takes a hushed and reverent tone and uses excellent vocal variety to hold your attention. Check the POL website later to see Henry in action--those videos and photos aren't yet posted. It's also a good source for poets whom you may wish to quote in your next speech.