Bekhorot 50: Mathing the Math (with Unholy Coins)

On the 5 silver coins for the redemption of the first-born son, and how they were valuated in other currencies. Which meant getting the math exactly right -- as the example of Rav Ashi sending to R. Acha, who was a kohen, and they need to send back and forth to get the currency exactly right. From the money/coins of the pidyon ha-ben -- when the dedicated coinage to the Temple, there was concern that the Hadrianic and Trajan coins would be all mixed up and profaned that money. Then a verse in Ezekiel calmed the sages down, because it was no longer considered to be consecrated. But there's so much gold and silver in the world - how would this restriction help?!