The idea of a RPG Unique Item in an online RPG is pretty cool. There is an item that there are no duplicates of. You are the only person who has it. It does something cool and you are the only person who can do something cool with it. This creates a sense of realism in your MUD or online RPG in that there are not two hundred Hands of Vecna lying around. It also makes your players feel like they own something that is of value, and special.
However, this presents one major problem, what do you do with a players artifacts when he seems to have stopped playing but has not given up his unique items? He may just be on vacation, so setting up a script to remove all of his uniques when he is out for a month is somewhat unethical, not to mention unrealistic.
The first solution is to reset all of the items unique or otherwise in the game at once every couple of weeks. However, this removes the sense of ownership and the sense of realism, so lets compound this problem by forcing the problem to be resolved in a situation where equipment and items are not reset periodically.
The next solution is, of course, mud rent. Simply, rent forces players to pay a fee for holding equipment while logged off.
But lets further compound the problem by saying that we aren't going to have rent. We are highbrow goons and we think rent is a silly way to keep items circulating in the game. It has the nasty side effect of making people keep playing even when they aren't really enjoying themselves, degrading the community of the mud, amongst other nasty things rent does.