I believe that EX is in reference to those cheap boxes that do not hold the NM sheath.
I like to use the "accordian" method by which all conductors are folded into, and can fold out with, a device.
This is better done with a those plastic boxes that secure the cable.
If you really take a close look at those cheaper boxes , you could stick your pinky in it, even with a cable in the KO , evident by the last sentence of the EX.
The end result of this style box is an accumulation of dust, dirt and even larger debris over time.
This alone should constitue an "unused opening to be sealed", but that's another thread
[This message has been edited by sparky (edited 02-15-2001).]