Several people have pointed to Greyhawk as being an example of a campaign that is "metaplot free," and I disagree with those people. Quality of the setting aside (I still think it is the best), Greyhawk has as much a metaplot as any other campaign setting. Need I say From the Ashes, anyone? That supplement alone can be considered one of the biggest changes to a campaign world in D&D history. Let alone Gygax's destruction of Oerth in the Gord the Rogue novels.
And you know what, none of these things has had a negative impact on my game. I have run both pre and post FTA campaigns in Greyhawk and I have both used and ignored the Gord the Rogue novel stories in my campaigns before. My players and I have enjoyed them all.
I imagine no DM would have a problem with a metaplot for a campaign setting if the metaplot followed exactly what that DM was doing in his own campaign. But the nature of the beast is that such a thing is impossible. And if you truly want your world to be alive and for there to be a REASON for new sourcebooks, then a metaplot is a necessity. You can't expect them to publish books on a campaign setting encompassing 20 years of history in which NOTHING happens, just to avoid stepping on any individual DM's toes.
Ok, so a sourcebook comes out detailing a war in a region of the world that your PCs just negotiated eternal peace in. Big deal. Comic books have retro-active continuity all the time, I'm sure your campaign could stand a little. And if you simply can't stand the existence of the product that describes said war and can't find a way to ignore it, have your PCs called off to another part of the world to continue adventures there. But just continuing on the way you have is not going to crucify the setting just because some novel or sourcebook has a different history than you do. If your players complain, state that there are an infinite number of universes for which an infinite number of possibilities exist and in the universe their PCs are located in, the war never happened. So you can't use the sourcebook in your campaign? Boo-freakin'-hoo. You might have been all excited to hear about a Kingdom of Bloodskull sourcebook only to get disappointed when you heard that they killed off its king, but for the other 99% of campaigns out there that probably didn't adventure much in the Kingdom of Bloodskull, the sourcebook offers the chance to explore new ground.
Anyway, the point is campaign sourcebooks cannot be viable products if everyone expects the designers to either a) abstain from a metaplot or b) adhere strictly to your home campaign's history. Enjoy the sourcebooks for what they are: the opportunity to add dimension to never-before-travelled ground in your campaign or an excuse to start a new campaign. These worlds have to have some action in them for people to continue to be inspired to start their adventure there and just because the designers have a different vision for the setting than your own is no reason to claim metaplots are the devil. They can't appeal to everyone, but I assure you a generic product with no action and no pizazz would die off quickly and have no pizazz. Either that or the designers would have to suspend themselves in time to prevent changing anything about the setting that the fans love, which would quite frankly become boring quickly. People are already complaining about the detailing of roof-tiling in the Realms. You have to shake things up every once in a while to keep things interesting.