The main objective of the rootkit is to do bad stuff or allow someone to do bad stuff to your computer. The second objective is for the rootkit to be able to hide itself and totally conceal its tracks (25-odd years of experience means people are pretty good at it by now ), which is why it's probably more efficient to just wipe the drive and reinstall.

As far as I know, rootkits tend to install themselves in low layers of the operating system like the kernel or the OS layer. If you switched the drive over to a new PC as a slave then since the PC is booting from a different kernel/registry/OS, more than likely you'd be able to clean the drive without having to worry about stuff jumping around the place.
That's about all you'd gain though, because you'd still be trawling through hundreds of files without knowing for sure what to look for. You would have to export the registry from the infected PC and search it for bad keys and again, you couldn't know exactly what to look for. The registry would presumably contain a few null keys as well, which you can't see, but have to find anyway.