The Relevance of Adventures
Glyfair, you hit the catch-22 squarely on the noggin with regard to the problem publishers face with adventures. Portability/adaptibility vs. setting specific: lean too much either way, and, yes, a publisher potentially loses a significant segment of customers. Necromancer's success, I believe, lies in how its adventures negotiate this catch-22 (i.e., primarily dungeon crawls with little truly specific setting information) -- but also in its rather well established fan base, which it captured early, held on to, and increased. The catch-22 is also an issue that we stay highly conscious of at FDP, though we tend to lean toward a bit more setting detail than Necromancer, I think.
Despite this catch-22, and despite the now incredibly varied D&D games and gamers out there (many of them sufficiently armed with a plethora of rules extensions from an abundance of d20 products), I would argue that adventures still have relevance . . . and that their relevance may increase.
True, in the past, only TSR (and WotC) published material for D&D, and so adventures represented an opportunity, as johnsemlak notes, to expand the game. Their importance and their ability to generate a kind of common D&D experience among players and DMs thus definitely contributed to their significant place in D&D history.
Today, of course, the field stretches farther and wider than maybe anyone ever anticipated. The current trend focusses upon The Crunch -- i.e., tools that DMs (and players) can manipulate as they see fit to build adventures that best suit their style and their players. Published adventures do not immediately present themselves as having The Crunch, so perhaps they suffer in consumers' eyes. Unlike the supposed halcyon days of ToEE and GDQ and ToH, adventures are small, small fish in a sea teeming with more recognizable species such as campaign settings, class books, race books, rules extensions, and so forth. Adventures today cannot possibly reach the level of common experience granted their ancestors; moreover, expansion of the game occurs through those other, bigger fish that I just mentioned.
The solution involves a little Darwinism: adapt or die.

One means of adapting relates directly to the whole genius of the OGL: adventures can take advantage of a wide array of rules extensions for several types of situations (for instance, Bastion Press'
Alchemy & Herbalism or FFG's
Seafarer's Handbook), which, I think, can certainly fuel creativity and originality. Another means of adapting becomes revising somewhat the very notion of what constitutes an adventure product -- in relation to the sorts of (d20) products that garner the most attention (again, those bigger fish from my previous paragraph). Necromancer Games'
The Vault of Larin Karr sets a good example: a dungeon crawl adventure contained within what amounts to a ready made valley setting of a few villages and several interconnecting plots between different NPCs. Think of all The Crunch you get in such a product: villages, NPCs, a wealth of locations, a small section of the Underdark, and so forth. In the future, then, as we hit a kind of supersaturation point of crunch-based products, adventures -- the good ones, the ones that adapt and innovate -- might get a second look . . .
because of the variety of "crunch" they offer.
I have to disagree strongly with Joshua, then. Such a view of adventures is awfully reductive. Granted, some adventures do amount to nothing more than a series of rooms or force a specific plot upon the players (and the DM), and those are the poorly written, poorly designed adventures. The good adventures do not fall prey to such traps, likely because they rely upon a strong, engaging story and a design that accounts for player choice. (These are also adventures that probably saw some decent playtesting.) In any event, Joshua's view of adventures also discounts their current and ongoing evolution.
So, adventures can and do have relevance by better meeting the needs and expectations of the d20 consumer. Two of FDP's adventures this year will approach the "mega-" variety but also include much more than "just" an adventure.
Gates of Oblivion will give you a small planar setting, for instance;
[As of Yet Untitled] will provide a detailed coastal town, elven town, and "underdark" aquatic setting. Both adventures will be using OGC from other companies' products, and both will contribute new OGC (templates, PrCs, magic items, spells, rules, and more). Certainly, all of this Crunch will mean little if the stories themselves do not capture the imagination, but I think good stories are a hallmark of FDP adventures.

In any event, these two adventures are just examples of what's in the works at FDP. I'm sure that other people can think of adventures already available that do similar things. (In fact, one could look to Monte Cook's "event books" as an innovation upon the adventure format . . . .)
I like to trust in two possibilities with adventures: one, that they can give us good stories; two, that DMs are creative and flexible enough to find ways to adapt published adventures to their campaigns if they are inspired to do so. Criticizing an adventure for being "useless" because it needs to be (heavily) adapted to a certain setting or campaign might be valid in some cases. Treating this situation as a universal "problem" with adventures, however, sort of cuts one off from the wealth and innovation that adventures do provide when done well.
Gotta go!
Take care,
Mike
P.S. tf360, I think you are exactly right on both points. It's also why we do see situations now such as Mystic Eye doing adventures for FFG's Dragonstar setting. Other such deals are cropping up, too.