Hussar
Legend
I come out in a slightly different place.
I think the bits of specific fluff and links are useful. Useful to lazy DMs, useful to newbie DMs, useful to big-boy DMs who want to run with it or tweak it. I'd much rather it be explicit.
What's not as useful -- to any of those groups -- is making it a default assumption of the game. It might not be hard-coded (which is better than it otherwise could be!), but if a new player is going to encounter the story that jackalweres are from Pseudo-Egypt and are linked with the god Set, and their reaction is, "I'm confused, I thought they were loyal followers this demon Grazz'zt?" (or worse, "No, they're made by this Grazz'zt thing, says so right here."), that's annoying. That's less modular and flexible than it could be. That throws up roadblocks (and it's unclear to me what benefit those roadblocks offer -- mearls insists that the stories the designers make have a point, but why they need to be the game's default assumption eludes me).
I like good, specific fluff. What I'm not so enamored of is training players to expect a certain story when D&D is a game of imagination and creativity, about forging and telling your own stories. Part of what makes DMing fun is finding out what jackalweres are in your world, what makes them interesting to you, even if it's just selecting from a list of currently-existing jackalwere stories. A game that says, "Jackalweres are X" makes that fun harder to have, since it gives players an expectation that you need to contradict, rather than just giving you an idea that you can use.
Now here's a question. You and I and others went several rounds over Planescape lore and how designers should not make any changes which negate existing ps lore.
How do you jive that with what you are saying here which is that lore should be fluid and easily changed?