One thing that the author is missing in the case of iMovie HD and iDVD HD is this: future-proofing.
While most people indeed don't have HD camcorders yet, they will in a few years. And guess what: their computer and its software will be all ready to handle it.
Given that most Apple owners keep their systems for several years -- much longer than the typical PC owner -- it's shrewd to tell them that they can buy a Mac with confidence knowing that it won't become functionally obsolete before it becomes physically obsolete.
Macintosh: The Computer for the Best of Us?