I used it in the early game when I had too many materials and wanted more marks to buy stuff. Later in the game you realize the importance of materials, but if I still needed a quick few marks and had extra materials of a type that I didn't need at least I had the option to trade. The only reason why I would see this option being removed is if the code only allowed for 4 placeholders under the specific vendor - meaning it was either marks for materials or materials for marks because the game couldn't support all combinations (8 total).