For example, in user32.dll is the function RegisterUserAPIHooks. That is totally undocumented.
However, it is one of the many undocumented functions that only exist to allow components of windows to talk to each other (in this case it exists only to allow uxtheme/comctl32 to hook user32 and do non-client-area themeing). The question is, do we implement such functions?
Well... If it is exported by the DLL then it should be exported. But if it is undocumented I don't think it should be implemented until an application is found which uses it. Not having a RegisterUserAPIHooks implementation doesn't make ReactOS run less windows programs. I would suggest something like introducing a "UNDOCUMENTED;" macro (like the UNIMPLEMENTED; macro) to track down used undocumented functions...
Sebastiaan