The big deal, here, is that I'd STILL be able to boot up my computer using my backup hard drive in the event that my primary hard drive died.
Just using XP, I can't do that; XP will not let me, which is why I was offline for ten days when my old 120GB hard drive failed. Yeah, I had my entire system exactly duplicated on my 160GB backup drive -- that's called an "image" -- and you'd think that I could have just swapped out the hard drives, set the BIOS settings to the new drive size and booted right up just like always.
XP doesn't work that way, though, and if you try that you end up with a perfectly good hard drive filled wit hdata and applications safely tucked away inside of a totally broken installation of XP that petulantly refuses to work.
Setting up the dual-boot would still leave me able to boot up in Linux and repair my crapped out XP partition from there.
Now for multiple hard drives, I'm not sure. There are a couple of guys here that are far more knowledgeable--they can probably give you more advice than I can...
(All my experience has been with one hard drive...)