As much as you want something...once you have it you will lose interest. What keeps people going in a pve, grind oriented game? The answer is that people want to get something so they play until they get it. If they opened up trading most people could get what they want in a matter of days. Trust me. You don't want this. The game would be empty within a month and the lost interest would result in the end of a potential ten year plan to produce destiny content.