[pdonis]

> I suppose that's a degree removed from printed bills or minted coinage

Yes, because the "value" of those assets is just a guess until you try to sell them. It might be a highly educated guess, but it's still a guess. And the time the bank would need to sell them--because it needs more cash due to an unusually high demand for cash withdrawals--is precisely the time that your educated guesses all go out the window: you're selling not because you're making a trade to improve your rate of return, but because you need cash, now, and buyers will know that and will gouge you on the price.