The same is said for a centralized economies in MMOs. In a centralized MMO economy the developers set the tone on what players can do and what players need and want. In regards to what players want or need, developers can easily guess that players will need a mount. But the question will be just how much should a mount cost?
Their initial guess may work as they believe that a mount maybe worth 100g (gold). They may believe that 100g for a mount is enough to satisfy and balance the money supply, meaning the purchases of mounts (along with other purchases) will offset the influx of money being created by players. But in reality the developer's guess in the long term eventually turns out to be wrong.
Why? Well it's hard to determine how to market will react in the future. Many players may find themselves not participating in the purchases of items that are bought or sold in the economy; however, they still participate in the creation of money through the means of pve and pvp activities. In simple terms, players are no longer doing activities that causes enough of their money to leave the game faster than they can create it. Thus the result is inflation.
A scenario like this would only work in a setting where the economy is centralized or not player made. Most consumer demand is supplied by the game (or developers) in the form of quests or drops. These activities results only a small amount of money to leave the game.
In a setting where the economy was player made and X amount of money is set, allows for market stability. How so? Well for one, the developers are no longer trying to determine the price of items sold by NPCs and two the players are now able to set the value of the items they're buying or selling. In simple terms, the market is setting the price of goods and not the developers, this will allow the developers to focus more on gameplay activities than trying to balance an economy.
I know, some may wonder what happens if a few players were to collect most of the money? Well it's not really possible because the truth is all players will need to spend money to perform any type of activity in the game. But even if this were to be the case prices will just simply re-adjust due to the adjustment of money circulating in the economy.
In the end, a set amount of money that will minimally fluctuate when players join or leave the game, allows for consumer confidence. People will understand and know that their will remain roughly the same all throughout the game's history. And if players understand the value of their currency, they will be better able to determine the value of products. And as I stated before, this will allow the developers to worry more about game mechanics outside of the economy.