Because you can't control everyone else in the group, and they will need everything they can and then try to use the stuff they don't need for trade. If you try to be the good guy and pass on things you don't need, you're gimping yourself, because they won't return the favor.
You must assume that everyone is going to take advantage when they can, and do the same by rolling need on everything. If you end up with loot you don't need and nothing to trade it for, give it to someone else at the end of the run.