I assume both: Rowling never thought of it, but if it were brought to her attention (and she considered it worth bothering about), she would probably declare that the Goblins had some clever witchery (or goblinery) that makes it impossible. So I proactively make that assumption for her, while still doubting that she ever thought of it.
More plausible explanation if you need a fanboy defense: Nothing in the text ever says the coins are pure gold and silver. They could work just like Muggle money and when people refer to them as silver and gold it isn't any more meaningful than calling US pennies copper.
The fact that coins are stored in large vaults and apparently aren't earning interest does indicate that there are aspects of the system that don't work as the modern muggle system does, but there's not a strong argument that the coins are pure metal.
Update: Discussion has moved on to a new thread.
The load more comments links are getting annoying (at least if you're not logged in), so it's time for a new Harry Potter and the Methods of Rationality discussion thread. We're also approaching the traditional 500-comment mark, but I think that hidden comments provide more appropriate joints to carve these threads at. So as of chapter 67, this is the place to share your thoughts about Eliezer Yudkowsky's Harry Potter fanfic.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six. The fanfiction.net author page is the central author-controlled HPMOR clearinghouse with links to the RSS feed, pdf version, TV Tropes pages, fan art, and more, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: