When would the Windows environment variable be HOMEPATH=\

HOMEPATH can be set to an equivalent of either %USERPROFILE% or the root of %HOMEDRIVE%. For scripts that you're writing on DOS/CMD on modern versions of windows (XP/Vista), you should use %USERPROFILE%.

That said, if you're writing an application (or vbs, etc), get the shell folders list from windows, as it is (generally) more accurate/reliable than %userprofile%.