I know what you're talking about, and it's used to great effect without being a boon on non-paying characters in PSO2 where they keep introducing new types of currency. Seems like they do it to effect inflation, it would be useful even if the game doesn't even involve microtransactions. I like having different levels of investment for the player, stuff like this is even done in a few zelda games. All that really matters is if it's fun or not, but developers of these types of games with microtransactions are using feedback loops to take advantage of weak minded players which I think is a bad practice.
I might be a hypocrite since I run bomy island which was a server that focused so hard on the value of currency as a main gameplay element that players were trading real life cash for it in 2001.