Merlin Mann's really good podcast on Presentation Culture is a reminder that there is an element of theater and show to all public speaking. Whether you use PowerPoint or not (I rarely do), good lectures, talks, and sermons need a sense of energy, a connection with the audience, and a strong ending. Without some sort of verve and gravitas, the lecture will limp to its end, especially in our culture and its endless appetite for entertainment. When you've reached a high point, even if it's a few pages from your envisioned end, it's best to end there.
In this vein but some 20 years ago, my college philosophy professor and mentor made his lectures quite dramatic, so that even those who didn't follow the ideas would have a sense of the drama of ideas that were being explored. The classic book on public speaking, I Can See You Naked (seriously), also encourages a dramatic flair that connects with the audience. That doesn't mean affecting Shakespeare, being smugly trendly, or pandering to your audience, but it does mean seeking out ways to have your method match your material, the emotions and the intellect, so that the students or parishioners can connect as well as understand.
You have to respect the show and the theater of public speaking, in order to be effective. Otherwise, you're left being Professor Binns, Harry Potter's History of Magic teacher who died but just kept on lecturing in his dreary, bored voice. No one wants their chosen area of expertise to become that.