The issue this would resolve is that on a multi-language website it is important to show the currency for that country. Many of our people wanting to give donations question when it is not in their currency.
With regards to the gateway, ie PayPal, this accepts payment in whichever currency has been chosen.
Also, I have found that changing currency in the setting, just changes the currency notation and any values that already stored in the Donation section switch to that currency, but the value remains the same.