Something occurs to me regarding sales of adventures/settings/etc vs. rules books.
Take a good long look at the forums under RPG Forums. We've got:
General Discussion
D&D Rules
House Rules
D20 Modern, D20 System, & OGL Games
Rogues Gallery
D20 & Open Gaming Publishers
Of note: there are no setting-specific forums, but there are two that are rules-specific.
Now look at the topics in General RPG Discussion. As I type this, the first page of topics has two, maybe three threads that are setting-specific.
(The d20/other forum seems to have the most setting-specific threads, but at a guess, that's because of the higher percentage of d20/other game books that are setting-specific vs. d20/d&d.)
Some perceptions I've run across:
For setting, what the GM says, goes.
If the GM can't come up with a decent setting -- either by using a published setting, mixing & matching from multiple published settings, or creating it from scratch -- someone else should be GMing.
Rules that players come up with are suspect.
Rules that have been published are potentially viable.
One GM quote I've heard, regarding rules: "If it's in print, you can use it." That went for classes, races, feats, whatever.
The same GM disallowed what seemed to me to be a somewhat-underpowered prestige class, because it had been created by a player. The player (not me, btw) had deliberately underpowered it in hopes of getting it approved.
So yeah, people are more likely to buy rules-related materials than settings.