Your argument still doesn't conclude that trading should be implemented. Trading would be even worse than random loot drops. If you get a legendary/exotic for another class, you dismantle it for shards(Or you can save it for an alt). The point of engrams would totally be flushed down the toilet, if all you had to do was trade your exotic hunter chest piece to get the exotic warlock chest piece, that you were hoping it would be in the first place...