This change seems to be part of the “closing” of the Kindle, where a sleek form factor trumps functionality. If the Kindle is to become the go-to reader of choice, it needs to be more rather than less useful; the device does not yet support folders, so keeping things on different chips was one potential way to store and organize your library. This removal of the SD slot also eliminates the possibility of a third-party vendor (I can hear the boos and hisses from Seattle) offering formatted books on a chip to readers. With 1) wifi that is hard to turn off and 2) no chip slot, you are pretty much left with the Kindle Store as your source of books.
And this is as it should be–in a retail universe.
But for schools, we need something more adaptable to different situations and uses. To the extent that the DX follows the design of the K2, it will fall far short of its promise as a device that could make sense in a classroom.