I think it's a pure anachronism at this point: it makes more sense to me in the era where small drive sizes meant a lot of professional users relied on external drives and “just upload it” wasn't going to fly with a 9600bps modem, not to mention hardware costing more and failing increasing the need to move drives between machines simply to get back in service.
Or until the host machine dies, in which case they have to be readable in another machine.
(Happened to me recently: the laptop's motherboard stopped working, I used its disk with an external case and an older operating system version while I waited for the warranty replacement.)
The article was pretty clear that the context they had in mind for this requirement was servers in a data center, not your home machine:
> Without that requirement, a hard drive might be usable only on the system that created it, which would create a major obstacle for data centers (not to mention data recovery).
Keep in mind they still thought they'd be targeting Alpha processors as late as the Win2K RC's.
That said, it would still be one hell of a weird edge case to need to take a drive out of an x86 Win2k server, drop it into an Alpha Win2k server, and still care about its contents (vs wiping it for a newly provisioned host). But when you are writing OS filesystems, you have to care about edge cases... especially edge cases that may apply to thousands of racks worth of machines.
I can imagine the reverse being more common though - I worked at a shop around that time where we had a handful of very expensive Alpha NT 4 (and VMS and...) boxes, and a lot of x86 NT boxes. I could imagine the magic smoke being let out of an Alpha and having to drop the drive into an x86 box for data recovery.