As this happens and markets gain knowledge of and confidence in the regime, exchange rates should become less vulnerable and less in need of defensive capital controls. In the meantime, countries should be ready to move quickly away from defence of an unsustainable rate (which obviously requires fine judgment about “sustainability”).
Allowing exchange rate movement (even if excessive) will, generally, be preferable to exhausting foreign exchange reserves defending a particular level of the exchange rate. (c)  Those countries where the case for a fixed rate is compelling might want to consider the rival merits of some form of strong policy commitment - a currency board or dollarisation.