werk said:
With a published setting the DM can add or remove most anything that they don't like (I removed the uber NPCs from FR, NBD) because there is such a wealth of material to use.
Published material is guarranteed to contain no mysteries, because once it's published it's not a mystery (and if your players aren't reading the published information then they can't very well be building a background on it.) So really the core engine of the campaign still has to be developed by the DM. Campaign settings (as opposed to adventure modules) contain some of the most superficial and irrelevant information in DnD anyway.
Secondly, if you're adding and removing things from the published scenario then how does that help the amateur novelist and his 18-page character background? What if you removed an element (like Elminster) that was cruicial to his character? Now it's about the DM having to communicate his particular vision of FR to the player, which has the same problem as the homebrew. I have to ask you, as the DM, what parts of FR you're using and what you're not using.
And it's worse if the campaign has any sort of longevity. A real campaign world is going to have organizations change, mysteries are revealed that changes the truth behind published information etc. Maybe Thay gets taken over and turned Lawful Good. That pretty much renders irrelevant any future modules, novels, or setting expansions that use Thay in it's original form.
Granted, those changes are pretty monumental, and are rare. But subtle changes might not be. Whether because of differences in interpretation, or development in a game, the way a particular organization or NPC behaves in a DMs vision of FR, versus the way they acted in the latest FR novel that a player built his background on, could be different enough. Basically, IMO a player hardly knows anything about a FR campaign that's more than 2 years old. His character background is unlikely to be compatible with the threads of interest that the DM has established for the game.
I'd bet a lot of DMs aren't running a game anywhere near Waterdeep, so of how much use is my encyclopedic knowledge going to be as a player? What if the Knights of Silverymoon aren't as "silvery" (or whatever) in that campaign world as they are in the novels? As a DM I might actually have to do more work to break the player of his preconceived ideas than I would to educate them on a homebrew order of knights.
It also introduces the problem that I might not be listening as carefully to what the DM is saying because I already think I know the setting. I might have read enough FR novels to know that Elminster is not the advanced agent of a demonic invasion on the prime, so I'd be likely to miss the clues. And any player predisposed to see "ego" involved in a DMs decision to run a homebrew is unlikely to look kindly upon me turning Elminster into a demon. I'd probably have to deal with comments about "ego" in that case as well.