I preach (yes, preach) the benefits of wireframing/sketching using pencil (or erasable pen) because of the flexibility and quick changes that can be made with that approach. Tracy Osborn, the author of Hello Web Design (a book that I have previously used as my COM 366 textbook) and The Power Of Pen And Paper Sketching writes similar reasons as I teach to my COM 366 class at Illinois State University.