It's only used by the volume portion of UNC paths. If you were to mount a Windows share onto a unix system, I don't see why you'd use "\\" (or "//") anywhere. For example, if \\server\docs was mounted as /mount/docs, I presume \\server\docs\some\path would be addressed as /mount/docs/some/path.

"Files and Filesystems" is correct. You shouldn't assume you can't. That doesn't mean you never can.