[I agree that item sharing would be similar in effect as save scumming etc. It'd twist the gameplay.
I think Item Sharing would have a more detrimental impact than using saves to re-do a part of the game. The latter merely allows a person a second chance, so to speak, thus the outcome is at least something that's possible (even save scumming to get a rare drop only tips the odds), in the first, situations never intended in the game would be commonplace. For example, a player could have made a different choice at the quest dialogue, or could have gotten the rare drop the first time; but, a player would never start the game with end-of-game gear and mass amounts of gold.
Also, given key items and quest items overlap, it would be possible to create a situation where a player has gear from two different quest outcomes.