That's... nice, but doesn't have any bearing on the topic at hand. Unless I'm mistaken about the point you're trying to make, in which case, please correct me. You said platinum is useless, and now you're saying it would be useful if not for its rarity. So, thanks for agreeing with me, I guess. To make some broader points, I would argue:
- Precious metals are precious because of their rarity, so it's moot to argue in a discussion about currency that a metal has less economic value because it doesn't do anything besides being rare.
- If we could set aside the issue of rarity because the worlds we create have metal rarity as we ordain (with some notable exceptions, like historic or post-apocalyptic settings), then there are other factors that make a metal useful as a form of currency. Non-reactivity is very important for both gold and platinum, to ensure both purity and longevity. Density could be an important factor as well, for the logistical considerations involved in its transportation in large quantities. And it's not an objective value, but prettiness undoubtedly plays an important role in how much value a society ascribes to something ("Ooh, shiny!"). I'm sure there are others.
- All metals have viable uses aside from rarity and non-reactivity... I assume. I'm not a chemist, so someone please correct me if I'm wrong.
Ok, will try to tie it in again with the main topic at hand. But my point is Platinum really has no place on a fantasy economy, it being extremely overrated as a precious metal and not being as desirable on it's own as gold, silver or bronze. Granted Silver and Gold have always been rare in the western and ancient eastern world, but that isn't the only reason they are valuable, they are ver desirable by people, the moment a giant mountain made of silver was found in the 17th century it didn't became less valuable, it just caused inflation (as it happened with the gold fever later on). On the flipside Platinum has more industrial uses and it's price drops wildy whenever demand is reduced, if we suddenly found a mountain made of platinum its price would plummet making some goods cheaper, without otherwise affecting global economy. (In the same way purple clothes became dirt cheap when anhilin was discovered) . Some things are valuable because of their rarity, some other things are valuable because they are just desired that much, real world silver and gold belong on the later group, fantasy gaming silver and gold, not so much.
This is because fantasy gold and silver are 1) extremely common (more so with the gold standard) 2) not having their desirable and intrinsic properties being perceived 3) not having other uses beyond being currency (wouldn't it be cool if you could use the gold pieces from the last raid to cover the hilt of your sword and ornamenting your armor? or use it to have the local jeweler craft a fancy holy symbol? In fact I would venture Mithril comes actually closer to be the real world gold analog in D&D because of this).
Inflated prices made sense on both the original D&D and the first AD&D, the whole point was more about a group of gold diggers/adventurer/archeologists/grave robbers on a frontier setting (basically old wild west, except on medieval fantasy land). However it sets a bad precedent when it comes to gaming outside that gamestyle, which is far more common now than it used to be, and it has indeed shown a slight and progressive amount of inflation over the editions (and sadly it hasn't been enough infaltion to keep up with treassure leading to the aforementioned "I don't have anything to spend this on")
That is why I like silver standard over gold standard, but not only that, gold rewards have to be reduced accordingly, and more uses for gold have to be deevised (like using it to craft jewels, cover gear and stuff). And even then it might come to a point where a higher value metal is needed (at that point I suggest mithril instead of platinum, it makes more sense and gives it more flavor)
