Blimey, you kwow it's bad when even Merric says it's weak sauce!
As others have said, Age of Worms had something in the form of the Overload and Dungeon showed the Savage Tide Adventure Path summary before running it. D&D campaigns being what htey are, it's very likely your players will go off-base and you might need to stall a few sessions to get them back on track - a GM really needs to have a rough idea of where the end point of the arc is.
That can be in more general details if they really want to preserve some mystery - for example, "extraplanar battles with a demon lord" or even just "evil outsider invaders using an artifact" is different from saying "Orcus is here, and he's got the Rod of Seven Parts". But, to be honest, I don't think there should be much in the way of surprises for the GM anyway - at the very least, I wouldn't want to find an NPC bumped off several parts ago was actually the grand master of the plot and that there isn't an obvious replacement. Any game in front of active players will end up going into divergences like this, and the GM needs to hav e the safety net of knowing what's worth railroading or fuding over.
Of course, I should add here that I'm more of a single adventure than an adventure path man as far as Dungeon was concerned: I was kinda bummed that Pathfinder was just going to be campaign arcs, although the single GamesMastery adventures made up for it. So perhaps people who have actually ran Savage Tide or the like will have a very different opinion - perhaps it's OK as long as the adventure has appropriate sidebars and comments, or perhaps it's just a case of DM common sense to deal with these situations.
Regardless, though, Wizards answer does seem a bit uncool, and it does make me wonder if there's more to it. I would ahev far preferred if, for example, the issue was time because of trying to get the DDI up and running that they just say so.