"I would be very interested in seeing something about the various currency systems in games. In WoW, this would include badges of frost/triumph/etc, honor, arena points, and gold. Some of these are easier to exchange for other things (loot, other currencies, etc). Do the differences in the different currencies reflect the differences between the parts of the game that each serves? In other words, why not make everything gold-based?"I think the starting point for that discussion has to be to define what exactly a currency is. A currency is something which might not have any value in itself, but gains value by being accepted as medium for exchange of goods. Thus $100 in a shopping mall are worth more than a million dollars on a deserted island. Trade and currencies are one of the great inventions of mankind, allowing people to specialize in one activity, and exchanging the value of what they produce against the value of what other people produced.
The same principle holds for many MMORPG currencies. If you want a specific item in a MMORPG, in most cases it only drops at one specific place. Add a low drop chance and Murphy's law, and you'll be running the same dungeon 20 times before you find the item you were looking for. Items that can be bought with some currency have a wider flexibility of what you can do to gain the currency. So instead of running always the same dungeon for some item, you run a wide variety of random dungeons to get the emblems to buy some item. Or you do a wide variety of PvP activities to gain the honor to buy some item. Or you farm gold, craft, buy & sell on the auction house to earn the gold to buy some item.
That neatly explains why there are different currencies: The different currencies have different rules on how to acquire them, and sometimes the game developers want to exclude certain pathways to certain items. For example they might want that only PvP results in the currency to buy PvP items with. One big issue here is that many games have introduced currencies that cannot be traded, to avoid these currencies being trades for real money. If everything in the game could be bought for the same gold currency, people could buy that gold from gold farmers and buy themselves the rewards without having done the activity that is supposed to be rewarded. Thus they'd "skip" the content of the game, and end up quitting earlier, paying less monthly fees to the game company.