<shudder> Did I hear someone say "Windows XP"?
Indeed, there's no reason for it. Piracy is one thing; using your own copy of software on more than one computer is another. And for that reason I will refuse to buy WinXP, OfficeXP, or any other technology that does the same thing.
(NB: The academic edition of Office2K had the same "feature" on it; they were apparently testing it for bigger and better things--and finding ways to get around those of us who got around it.)
Personally, I see no reason why they don't offer a "family" license: use WinXP/whatever on all of your family's computers. You pay $30-$40 more than standard pricing, and we don't care how many computers you put it on, but you're on your honor not to put it on computers not in your home/in dorm rooms or apartments belonging to people whose permanent address is still your home.
Much as I dislike Microsoft in many ways, I think I might even be willing to pay the extra few bucks, just to make sure they're not out looking for me.
LL