i find interesting when you claim "The drawbacks those systems have in real life do not apply here." without providing not even an explanation...
I was thinking mostly of electronic payment systems like money cards or credit cards. These systems do not give you direct feedback of how much money you still have. Weather a payment with those is accepted or not depends on several further circumstances, and privacy ultimately cannot be guaranteed. All this does not apply to the monetary systems in MT - be they based on an electronic account or items in the player's inventory - because it's no difference either way.
the huge ammount of money accumulated in the server means that any old player can now buy everything, consuming all resources that are being sold.
But that is not what we see happen on servers. What happens is that the rich old players have pretty much of everything. Not only money. If those players would start buying in large quantities, they'd motivate players with less money to produce more for selling. But why would they? Their own chests are overflowing.
the normal answer is : increase the prices to compensate the increase in demand.
Where's that increase in demand coming from? A new player may need a few items here and there, but that's it. Hard to get rich on that.
you have EFFORT to make coins out of recovered minerals, money cannot be created out of nowhere (by script, or by mod command).
There's some effort involved in building nice shops and getting players to buy anything at all. After all they could dig the stuff themshelves. Money is just another type of ressource, like gold or mese. It has the advantage of making trading easier (same as with real money there).