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.
This would include small countries with a similar economic structure to the potential anchor country; countries (including larger countries) with a high degree of economic integration, measured by trade or investment flows; or those with a history of poor economic management and currency crises, where governments cannot borrow long-term domestically, or in domestic currency offshore.