I think the purpose of this exploit should be clarified.

This exploit originated in a twinking community - which explains why it exists. Twinks often wear non-primary armour types, such as cloth on Druids/Shamans, or Leather/Mail on Paladins.

The point of this is to transmog those armour types into another collected appearance of that armour type, as you'd normally be given the error message saying that "this item cannot be transmogrified" otherwise.

IE: A Paladin wearing Mail Legs could transmog to another pair of mail legs who's appearance he has already collected, but couldn't otherwise transmog from the interface due to limitations.
It does not allow you to transmog to items you don't own - such as a Corrupted Ashbringer if you don't have one.

So this has limited practical applications.

Want to wear Mage T3 that you already have on your Warrior for shits and giggles" Grab some random cloth gear from the AH and do it. Have a twink" You can now transmog non main item types. Want to conjure up a Corrupted Ashbringer and full Dreadnaught with the Contest Winner Tabard" Not happening.

...