You still didn't explain how Trading would break Destiny. There are plenty of games with RNG and a trading/economy system in place. Not a single one of those games is broken, only improved. Diablo 3 didn't work because it involved real money, not in game currencies. Even without RNG involved, being able to trade items would immensely improve the social aspects of Destiny. Out of heavy synth? Maybe a buddy of yours could toss you one in a firefight. Need Weapon Parts? If only those players who have way too many could trade them to those who don't.