Actually, the purpose of the ForceGuest registry key is to turn on or turn off the "classic" Security Tab so administrators are able to change the rules in Discretionary Access Control Lists. DACLs are associated with directories and files. The default for some SKUs of Windows XP is to make the security tab in Windows Explorer go missing. This has something to do with the "Guest" user being "forced" to logon in some peculiar way when a computer belongs not to a Domain but to a Workgroup. It was decided 10 or 15 years ago that having a Guest logon to a Workgroup posed a major problem if the Security tab were to remain available by default. So it was whisked away and the ForceGuest key was born with this lame "explanation."
This isn't a technical problem so much as another iteration of the inability of puzzler-style technologists to describe and explain what they decide to do and why. Now, if I want the Security tab to appear, do I want to force guests on or do I want to force guests off? I dunno.