Basically it's for having a working xp installation with synced my documents, to which i have all my saved files from different programs including my scanner, in different folders. Also I keep my downloads in my documents.
It can also be used for other means, use your imagination.
For instance you could sync all your programs provided they are installed on both drives. I don't think it would work with out it installed.
I haven't tryed syncing an entire hard drive to a slave drive. It's main purpose for me is in case of a malfunction that I can't fix within the xp install. I can go to the other install and see the differences in files as well as work with that install from outside of it instead of inside of it.
If I can't fix it, I still have a working xp install and reinstall another on the drive with the malfunctioning xp install at my leisure instead of a push to get my computer back up and running.
Defrag, checkdisk, tuneup 2007 and a host of other programs can be used to diagnose the failing install from the other install.
It's totally legal and even passes ms's rules.
I use older computers-p3's and my hard drive started making noise that i didn't like, so i started looking around for how to transfer to a new hard drive without having to call ms again.
And I found that there are numerous advantages to a setup like this.
I do not have raid anything. If it doesn't work on your computer, I don't know why.
You could boot a drive that is totally dedicated to a resource hog if you choose, that's just one idea, instead of having to close multiple programs, and when you are done reboot to your desired xp setup.
For activating the slave your need your coa #, and no problems, at least on my computer.