I can play the game without them, obviously. *shrug* I suppose that's the way I want to use the term, 'core.' The PHB/DMG/MM triad are sort of the essential elements of a d&d campaign, along with a homebrew campaign. I suppose one could homebrew all kinds of stuff, like monsters, so the line in the sand I've drawn is in some sense arbitrarily.
But, in my head, regardless of definitions promulgated by marketing, the sequels aren't core for me. I'll buy them solely based on their individual merits.