Imagine a tug and trailer hooked up, then imagine them moved a foot further apart. Now there are two choices to couple them back up:
extend the trailer tongue - ball is at the front of the one-foot extension
extend the hitch - ball is at the back of the one-foot extension
The two are very different, because the ball position is the pivot point, and that's critical.
Extend the trailer, and the tug has a longer lever to use to control the trailer; extend the hitch, and as described above the trailer has a longer lever to cause problems for the tug.
The extensions are a lousy and temporary way to extend the hitch, but even if they were done well, they would still be entirely undesireable from a trailer stability and suspension loading point of view. If you're using a one-ton dually to tow a teardrop, and you want to extend the hitch to allow room for a bike rack, then go for it; if you have a normal towing situation, I suggest extreme caution if considering a hitch extension.