I do not want to, but my kids do, a lot! They'll get multiple logins going, walk away, forget what they're doing, and then someone comes up and tries to shut down the system. Now, there's something like 3 or 4 logins running who knows what, because everyone has forgotten what they were doing, etc, but there goes the machine, shutting down.
This has happened so many times to us that I now routinely disable FUS, as a matter of course, because I cannot trust my family to just log in once.
And that is the reason why I want to know how to disable fast user switching.