I think multi-currency systems can be used to juice microtransactions pretty hard still. For example League of Legends gives players free "chests" and free "keys" as a slow-drip, and you can open any chest with a key to get some random loot. If your chest count doesn't match your keys you can buy more of either using real cash. So basically multi currency systems can be used to keep players intentionally in a state of imbalance. It also makes it so their money doesn't go as far in the game if you have to buy separate blue yellow and red coins instead of just yellow coins like it sounds like in your Hades example (just having 9).