I think coinage as a commodity will start to become desirable for the same reasons it has historically... High value, low logistical over-head. Also, I can only imagine that the changes to the trade system will increase the amount of trade activity your fort experiences, which all the sudden gives you impetus to truly get interested in trading... And when you start moving massive amounts of goods around, storage space and weight start to be considerations. Hence, a natural push towards the use of coinage.
The eventual return of the dwarven economy will more or less force the return of currency as well. I like the idea of an abstracted currency backed by a physical stockpile for intra-fortress commerce very much. So much so that I didn't realize someone had already posted it, and made a thread about it ^_^. But, I would like to point out that you would have to factor in a few things for this system to work:
One, deciding if the player can use the currency (IE the administration could borrow from the dwarves), and b) if a dwarf no longer exists, what happens to those credits? And also, the physical coins that back them?
The racial differences thing seems a little silly for a few reasons. First off, dwarves are the industrial powerhouse of every generated fantasy world. They have the best stuff, the most raw materials, and the least reliance on outside trade. When the time comes to trade with other races, dwarves are going to be the race that takes an imperial attitude, if anyone would. In this fantasy world(s), along with most others, dwarven greed has always been an important component of what it means to be a dwarf. Therefore it makes absolute sense that dwarves would be the center of the economy. Also, dwarves move the heaviest goods around. Everything they make is metal or stone. They would definitely value the concept of currency highly. The value of currency in this fantasy world, as I see it, must be determined by the value of its associated metal. Nothing else makes any sense for the time period, really. Plus, I think coins should melt to form the exact amount of metal used to make them, unlike other things. It just makes sense.
Trade with other races -should- be focused more around the acquisition of random trinkets or trade goods then useful industrial goods. As the world currently stands, those races are basically lesser. Which I don't see changing any time before magic rolls around, to be honest. Humans have numbers, but no steel, and elves are a bunch of tree hugging-hippies. Even if you added in silly rare items that different organizations requested, those caravans wouldn't become any more interesting to trade with. Just more of a chore. I would say that human caravans would be a great source of food, and elves can sell you (awkwardly) wood and animals. You'd probably want to save your coins for when the dwarves roll through, so you can buy the stuff that matters (to dwarves).