They took it out so that you can't buy materials and then trade them back for marks.
Example: spend 100 vanguard marks, get 200 Spinmetal, then trade that Spinmetal back for 20 crucible marks.
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They could have added a way to keep farming a part of the game. Open Faction vendors to accept both types of marks would have been any easy fix than to block access to 3/5 vendors unless you grind in PvP.
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I still farm. Mainly when I'm on those boring "do 6 patrol missions" and I'm saving up marks. I've got weapons to replace.