For a small state, the government does not have much practical control over monetary policy - or more accurately much [bi]constructive[/b/i] control - there is the Mugabe option - but the outcome is not particularly pleasant.
Controlling exchange rates (if you can do it) is a handy way of raising or more relevantly [bi]lowering[/b/i] domestic prices relative to the outside world. Practically what one is doing is adjusting the relative wage rates between currency zones.
Commodities that can be moved will tend to move to where they fetch most, while land and buildings might change ownership if they seem particularly good value.
However few states are large enough to get away with having their own unattached currency - something that is vulnerable to Forex speculation, market sentiment etc. Hence currency zones, using ‘hard’ currencies for larger transactions and/or pegging against a stronger state.
Probably getting the Thirld World to use just one currency would not make a jot of difference to the Thirld World - for a start any serious transactions are unlikely to be made in cowrie shells - and as others have pointed out Third World problems are caused by kleptocracy, geography and profligacy.