1 - it isn't obvious what it does (if a file has a symbolic link, then is the link the same file? what about a hard link?) 2 - it offers no performance benefit

1 - it isn't obvious what it does (if a file has a symbolic link, then is the link the same file? what about a hard link?) 2 - it offers no performance benefit on sensible compilers (which have a special case in them that can detect the typical header guard pattern and so avoid reading a header twice) 3- it is non standard, and the feature it provides is one which is easily implemented within the standard, where the only disadvantage seems to be that you need to type more characters.