KitKat will make your SD Card completely useless: per the Android API specification, apps can no longer write files to your SD card. And Samsung is following it. This only applies to dual-storage devices, i.e., devices with a user-writable internal flash storage AND a removable SD card.
Oops. You may want to hold off your KitKat update, if this turns out to be the case.
That looks like an older version of the truth. :-)
External storage access
"You can now read and write app-specific files on secondary external storage media [...]"
This means apps are no longer allowed to write anywhere on the SD card as they please, but are restricted to some app specific directory. And they have to use the correct API calls for that, meaning that with some apps you might have to wait for an update.
"Completely useless" looks way overblown. It's just file managers that become useless.
The old model, every app with access to the SD card could access all of it, was never a good idea.
Re file managers. Google could introduce a new additional permission for full access.
But what is really needed and was from the beginning are conditional permissions.
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