Unless you’re a extreme gamer, do CAD or video editing, most people who by or build top of the line have way more computer capability than they will ever use.
When I went from P3 to P4 I didn’t notice much of a difference.
I like lots of memory though.
Or unless you want to use Second Life, watch high-res video, rip your CD collection to MP3, browse Google Earth, or edit photos or video. That's a few tens of millions of users right there.
And, heaven forfend, some folks like to get a machine that will have the capability to run whatever some clever dude comes up with next year or the year after.