What is the problem with a "short arbor"?

[RRio]

I know this has been beaten to death, but what exactly is the problem with a short arbor?

[Sedalia Dave]

When the wedge is there hold the barrel on. For it to do its job it has to "wedge the barrel and arbor into a fixed repeatable position. It does this by forcing the barrel into contact with the end of the arbor.
When the arbor is the correct length, it sets the gap between the end of the barrel and the front of the cylinder to the correct distance. If the arbor is too short the barrel is forced back into the face of the cylinder making the cylinder gap too small. A small cylinder gap can cause the action to gum up as fouling is deposited on the face of the cylinder.
In some cases a short arbor will allow the barrel to contact the cylinder face preventing the cylinder from turning.

Some people try to compensate for a short arbor by not fully inserting the wedge. This can allow the barrel to have a slight amount of movement. With repeated firing this slight movement will cause the slot in the arbor and barrel to grow as the barrel is slammed forward each time a shot is fired. Eventually the slot will become so large that the wedge will no longer stay in the gun.