I think the problem with a lot of what goes into a DMG (or *the* DMG), is really just a collection of setting-specific facts that don't really have anything to do with the mechanics of the game. Much of what Hasbro publishes is setting-dependent, disguises are core rules, and this is increasingly creating problems for the game, as players demand that anything officially published by Hasbro is something every DM must allow. As an old-timer (playing since 1981), this runs completely counter to my understanding of what RPGs are, can be, and should be.
I'm in the process of writing an RPG system, and I do plan to start off the book with a brief introduction to what an RPG is, why you might want to play this one (Tapestry RPG System), what you are going to need to play it (Players, Rules, Randomizers, Recorders, Setting, Scenario, and Session), before delving into the specifics of PC creation and system mechanics.
And I do intend to include an example setting with example races/species/cultures, locations, items, spells, and a bestiary, I'm going to keep that section brief, because I want players to create their own settings, and not rely on mine.
It's not that I'm not sympathetic to the idea that the players playing the PCs and the player serving as DM/GM shouldn't view the Setting/Scenario/Session as a collaborative effort, but someone has to be the arbiter of creating a cohesive universe in which to play.