Given that the new installer now has the option to enable extensions that are located in $IP/extensions, the ParserFunctions extension should come bundled with MediaWiki.
This bug is not about including ParserFunctions in core. It is about including the ParserFunctions code in the extensions directory by default, which would allow the installer to enable it if the user chooses to do so.
ParserFunctions has become nearly essential for a MediaWiki installation. Requiring end users to install the extension separately is a completely unnecessary hurdle.