Drives ideally stay in a server machines until they die in a data center, and maybe even typically.. but MS could hardly only take into consideration what was 'typical' when it comes to things like this. And he's writing about engineering decisions made over 15 years ago here - around 1998 it certainly was more common to move hard drives around between machines. I probably did this at least 100 times just working for one company for a couple of years.
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.