M$ Legal here in AU and when I have made use of this in the past I have just Rung the M$ Activation Line and asked for a Activation Code.
They then ask lots of questions and I very much doubt that they pay much attention to your answers as I was asked about one Install which was a VL Product that I had running on 5 systems and the person at the M$ Activation Line started going on about Piracy and how I had just admitted to doing it. ;)
But provided that you give them something that fills the boxes on their computer screens they are happy. I very much doubt that they have any direct knowledge of what they do and that their only requirement is to be able to read a Screen and left click on a Mouse button when necessary. Like most entry Level Help Desks I very much doubt that the staff who man the Activation Center have any direct knowledge of the real requirements that M$ has.
For that matter the M$ Licensing Staff will give different answers to the same question on different phone calls. I've had examples where I have got different answers from the same person for the same question [i]about Licensing[/i] on the same day with only a hour or two between the phone calls. Even then I only ring them because I need a answer that isn't available on their web sites and from my experience I just now ring M$ Legal Direct and ask what their position is. It's far safer that way. ;)
But you are right you only need 1 Install Disc for XP or Vista if you want to use the Downgrade Option available it's just that most people who will be doing this are home users so they need a Install Disc of the OS that they want to use. Business should be using VL Product to begin with but even if they are not they only need the one install Disc to Downgrade as many computers as they want.
That was one of the major advantages of XP you didn't need a different Install Disc for the OS as you could use the 1 Disc with different Product Keys for that SubGroup of Windows. It made life for me much easier if nothign else. Instead of needing to have the owners 98 Install Disc I could just have a XP Home, a Pro, A Media Center and a Pro VL Install Disc and then select the correct install disc for that machine and use the Owners Product Key. Now that XP is reaching the Nonsupported stage of it's life I don't think that M$ will be overly concerned with Licensing Issues too much now. I know that I have used the one XP Pro Product Key to install XP as a Downgrade to at least 30 different owners systems without a issue from the Activation Center. But maybe to save myself more issues I should just be installing from a VL Product. :D
Col