You can do the dual boot that way, but I set the guide up as most people only have 1 HDD.. but if you have 2 or more drives, you can certainly install to a different drive, but for both OS's to be on the same computer at the same time, you still have to adjust Vista's boot record. That is what the guide is mainly about, how to configure both OS's on the same box without having to swap drives or unplug drives. It eliminates the need to change the Bios as most people are uncomfortable messing with that.
The method you prescribe would require that you install vista on 1 drive, remove the drive, set up the second drive, install the other OS on the second drive, and then swap drives out depending on which OS you want to use.
If both drives are installed, the computer will load the first OS it sees, then the OS will check for other windows versions, and that means Vista won't know that XP is there because the boot record is incomplete, and XP won't know Vista is there because XP doesn't recognize vista at all..
You left with swapping drives or changing the Bios every time you want to use a different OS..
The way I am set up, I just restart the system and choose the different OS.
Partitioning the HDD is just a way to trick the computer into thinking it has more than 1 drive.. as such, the guide will work if you have multiple drives.
It is conceivable that if you have several drives, you can put all the older OS's on your system at 1 time, though most people will only need XP and Vista for newer and future programs.. but Win2K3 and Win 2K should be able to install using the same method as the guide points out, you just have to change the name of the OS when configuring Vista so that Vista Boot Record will recognize the OS and give the selection at start up.