The transaction will be processed letting the issuer handle the exchange that the network (Visa/MC) provides, so if the rate is more favorable due to the foreign currency depreciating you would benefit. From the merchant's standpoint, it's neutral for the transaction if you don't take into consideration the cost of the chargeback.

While it isn't a factor for US cards, many issuers in other locations attach foreign spending bonuses that are only applicable if the transaction happens in another currency. I would still pursue the chargeback on principle.