"...from a Publishing perspective" sounds like this is directed at publishers. I'm pretty sure that's not the case, though.
If you will forgive me for comparing RPG (publishing) to buffet dining:
The core ruleset is like the plates and utensils. (Can get messy if parts are missing.)
Setting and adventure materials are like the tables and chairs. (Doable without, but perhaps not the most comfortable)
Supplemental materials, expansions, "gazetters", support of various types (errata, FAQs, message boards, keeping the backlist available) etc. is the range of food available at the buffet. Ideally, there is a reasonable selection of well prepared dishes, from which the diner may select that which is most appetizing. (Mix and match is nice. But you still have the option of cooking at home instead (and if you happen to be broke that's how it's going to be)).
That's my perspective on it.