With traditional instruction, once a book is written and published it is impossible to make changes.
With the rapid pace of technology, Authors find themselves writting books that are outdated shortly after their book is published.

Example: Dreamweaver MX killer tips -- by the time this book is released Macromedia is set to release Dreamweaver MX 2004 thus rendering this first book obsolete in no less than a year and a half. If this content was created as an instructional video most of the content would still be usable and modifyable to have a much longer shelf life.

In addition, what if you are an author and you find out that you made a mistake in your teaching, once the book is published you are out of luck.