Not necessarily the exchange rates are to great for example for 10 marks you get 20 materials. However you needed 50 material to exchange for only 5 marks plus reputation it wouldn't be economic to pay 30 marks to receive 5. Essentially you pay more for less
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with 100 vangaurd marks you can urn that into 20 crucible marks and 100 reputation. Why would you make it 5x harder for yourself to get crucible marks
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Finally someone bothered to do the maths