Mercurius
Legend
Well, I can't say for sure, as I couldn't be bothered to buy more books, but I've heard different. As an example, I read someone saying how the rules for vehicular travel (beyond horses..) are in the Adventurers Vault, or something else...
And the rules for pre-industrial lavatories will be in Adventurer's Vault 7, but so what? Again, the keyword is optional. Don't have rules for vehicular travel? Extrapolate or, in other words, make shyt up. Or, even better than pasting on yet more rules, just DM Fiat the sucker.
As a side note, what RPGs out there do you feel are truly complete in one book? I suppose we could cite the Rules Cyclopedia or various non-D&D games like Talislanta 4ed or Artesia or Eclipse Phase. But even with those games, or many of them at least, they come out with supplements. I just don't see how a WotC or a White Wolf could survive without supplements. What would they publish beyond the core book(s)?
Don't get me wrong, I don't buy most of the crunch-focused books (like the Powers stuff), especially with everything available on D&D Insider, but I don't see how WotC could possibly run a business--with a continual revenue stream--without the "supplement model." The only alternative I can think of would be a complete downsizing of the company so that you'd have a few people manning the ship until it got to the other side of the ocean (the next edition), with a long "archipelago" in-between which consists of freelance writers churning out modules and settings. Nothing wrong with that approach either, but it is long way from here to there, and it may happen eventually but it doesn't make the current model inherently wrong.