As of Patch 4.3 this is working as shown - the unused parameter has been removed.
This function will generate an error if the maximum macros of the specified kind already exist (36 for per acount and 18 for per character).
It is possible to create macros with duplicate names. You should enumerate the current macros using GetNumMacros() and GetMacroIconInfo(macroId) to ensure that your new macro name doesn't already exist. Macros with duplicate names can be used in most situations, but the behavior of macro functions that retrieve a macro by name is undefined when multiple macros of that name exist.
This function may not be called in combat anymore, as of Patch 2.0.