<rant>
This idea is complete crap. Here's why.
Say that he has NAV installed for one user and stinky Pete's AV and weather checker installed for another user. You can't run both AV scanners at one time so how the hell does this help you? You'll have to login and scan the drive under each user that has a different AV scanner.
Not that this even crossed his mind but some AV products don't play nice together, mainly because normal individuals don't go to the store (or Kazaa) and grab several AV scanners and install them all. Have you ever seen what happens when you install two AV scanners that add LSPs to the stack? I have and let's just say that you'll have the safest computer on the planet because you wont be getting out to the internet anytime soon.
</rant>