The highest value coin in the game. Platinum is amazing and yet not as entrancing as it should be. I personally like the silver standard that I see being used by a number of other people. In that platinum is not needed, oh lets slow down and take a look at that. It say not needed! Thats getting ahead of ourselves, why would platinum be needed in the first place, not that is a good starting point.
The common people of a setting will never see a platinum baring having an adventurer with one come into town. Of all the places the players might go almost no one will be able to make change for even one of them. This is like when America had the 1,000 dollar bill. The only people who actually used it where banks exchanging money between themselves. Now that would be a fine use for platinum, a coin used as a way to trade around money between things like kingdoms but that is not what it is used for.
A use for platinum may be as a non-existent coin. Something to mark in the accounting books various amounts like total earnings over a period of time. After all even if your goods are not all that much over a year the amount earned can be quite big. You don't have all that money anymore but at some point you had each bit of it and you need to keep track of such things to run a medium to big business and even a small one needs to do it for tax reason. But once again that is not what platinum is used for in game. After all you find it as treasure.
What good is a platinum coin? As a metal its not really used in the time period that D&D is representing. The first European reference to it at all is in 1557 and is first widely recognized as a metal in 1750 when someone presented a detailed account about it. This means that during the Middle Ages of Europe the metal itself is actually unknown. Also while it is very rare its actual uses during the time are, well its shiny and rare. So rare in fact that having coins of it is absurd.
In the end what I see platinum being used for is as a crutch. As written a gold coin weighs so much and is worth so little comparatively that to have a treasure trove truly worth it you need more then even an old school party with all the hirelings and such to bring it back if it was in gold. A platinum piece allows a treasure to have a high worth without being impossible to retrieve without multiple trips back to a place that is already explored. The later the edition the higher a level you are expected to get to so the more treasure you will find until it becomes quite unruly to use gold let alone silver or copper.
What does this do in the end? It makes it so that gold is to common and copper is worthless. Silver is useful but only for lycanthropes. Common people don't go around with a pocket full of gold coins, its generally going to be silver coins that they carry. This is true even in the later editions of D&D because in 3.5 a days worth of good meals is only 5 silver and if you only want common food it is 3sp and with poor meals it weighs in at a single silver. That means that a peasant won't even spend a gold for food in a week and yet the heroes are going around with purses bulging with gold. The amount of money a player gets between level one and two is enough to set them up for quite a long time of non-adventuring.
Platinum is fun and all but to finish I must say I would rather not use it at all. If I was to have a coin above gold it would be one of the special metals like Mithral where at the least it would be good for something as well as being rare. The best answer though is to use a silver standard instead of gold. A move to that would make gold actually be something once again instead of just money.