Motes and Strange Coins are mostly RNG based, short of doing the couple of activities that rewards them. If Vanguard/Crucible marks are used than players have a guaranteed reward from doing activities that are accessible to most players. I'm trying to come up with a more guaranteed way to accumulate the currency needed to obtain Etheric Light. Also, I'm much more likely to spend my Coins and Motes than I am my marks. I know this may not be true for everyone, but that's the case for me.