Right now, NO_WRKSUBDIR means that the extraction does not produce a subdirectory, and that everything is living in WRKDIR. It is problematic, because during the build of a port, quite a few files are created in there, and then, a stage directory, where everything is installed, and then a pkg directory where the package is created.
With this, NO_WRKSUBDIR will extract the distfiles directly into WRKSRC instead of WRKDIR.