There will never be trading in destiny under the current RNG system. Players would obtain items too quickly and there is Not enough content to keep people playing if they have no carrot to chase. A trade voucher system would allow people to gain items they really want even if the vouchers themselves were rare. The voucher bypass would allow you to control what carrot you capture.
Check out this Reddit to get insight on the psychology of destiny's loot system.
http://www.reddit.com/r/DestinyTheGame/comments/2itlvw/destiny_addictive_formula_detailed_by_bungie/
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