As a user looking for a report about a MediaWiki core bug, I want to quickly look for it without bothering about non-core reports or specific components/projects, so that I find what I'm looking for before I give up out of desperation.
Currently, I must type N MediaWiki core projects manually in the "Projects" field. (Cf. https://secure.phabricator.com/T3670#65849, first bullet.)