The choice of building reserves in a demonstrably harder currency is almost by definition not without costs in real resources. The budget cost of paying higher interest rates for the domestic borrowings employed to purchase lower yielding U.S. dollar assets, for example, is a transfer of real resources to the previous holders of the dollars.
The real cost of capital because of risk is higher in a weaker currency country. Countries with weaker currencies apparently hold hard currency reserves because they perceive that the insurance value of those reserves at least equal they re cost in real resources. Reserves, like every other economic asset, have value but involve cost.