I think these 'fire and forget' settings are going to be a long term mistake. These new one shot settings are lesser than their previous supported versions. And I am not even making a quality argument here:
There is now less setting available.
Take a look at the FR books. Two books to try and squeeze ALL of FR into? How can that be as useful for a DM as being able to, for instance, get a book that details Waterdeep enough that you could run your own campaign in it as written?
The same for Eberon. Detailed books available about Stormreach and Sharn, the special roles dragons play in that setting, etc...
Yes, DMs can fill in the material that they want. But these core setting books are just outlines of a setting with all the focus taken out. You now have less options available to you, no matter what your preference may be.
Previously if you wanted to run FR using just the core book as an outline, it was just as possible as it is today. But if you wanted details about a specific reagion or dungeon... you could find it! Playing in FR or Eberon, or Greyhawk etc had the advantage of a rich amount of details to use or simply inspire.
And also, they served as examples of how you could detail your own world! As much as I like my own imagination, I am not so overconfident that I would think I had nothing to learn from the efforts of other gamers, authors, and cartogrophers.
Now you only have the one option, if you are only using 4e material, And that is to use the campaign setting and fill in the blanks yourself. Some DMs may prefer it, but if you wanted more details? You no longer have that option.
There is less here, and how can that really be better than the alternative?
A common complaint I see springing up about 4e lately and the 'everything is core' stance to its books is that so many rule books are overwhelming the game.
The common (and I think correct) answer to that is that you don't have to buy/use them if you don't want to! Having more options, books, and resources available does not mean you HAVE to use them.
And if the above is true, then the same is true for settings. Use what you want, don't use what you don't want.
So if 4e continues these Fire and Forget settings, ESPECIALLY if they keep trying to shoehorn in every race/class/option from every other setting and PoLand assumptions to the point that they all seem the same, then I think they are making a rather large mistake.