Uhm, I don't know what you are trying to say here. The "Windows" user interface sits on top of the Windows NT kernel, just as the OS X user interface sits on top of the Mach kernel, and the Ubuntu user interface sits on top of the Linux kernel. Maybe you're trying to say that you need to understand how to use a command-line shell to truly "use" Unix? I seriously doubt that most Windows users today have ever learned how to use a CLI, even though every copy of Windows comes with one. Why should an OS X user have to?