renau1g did indeed understand my reasoning.
In a single party campaign, items may get traded among characters, to the benefit of the group. But once everyone has a +2 weapon, the +1 sword that the fighter used to use eventually gets filtered out as 1/5th value, as do any other +1 items that show up.
In a living game, once I get a +2 sword, I want to sell my +1 sword. If I sell it according to the standard game economy, I get 1/5th the value. If instead I sell it to a lower level character for say, 1/2 value; then he and I have both benefited. I have more money, I'm closer to my +3 sword. He has his +1 sword, and half the gold left over from what he would have spent if he'd bought it in a shop. Now we have two characters that are richer than they should be. Obviously not by much, and its not very important, but magnify it over all the players, over a long period of time and it could become significant.
The difference between a living campaign and a standard game is the possibility for regular interaction between characters of differing power levels.
Now I will admit that I do not own, and have not read the 4e DMG, so if the item distribution guidelines will prevent this kind of thing from ever being important, then I apologize for bringing it up.