Well, that may sound like a pretty silly question with all the Euro mess hoo-hah at the moment. With riots in Greece and Spain, and "austerity" such a dirty word, why would any country want to join the Euro? And to be clear, by joining the Euro, I mean completely dropping the AUD as legal tender, and making EUR legal tender in Australia. Well, 17 countries have joined the Euro. Why? Surely, a nation must have a good reason to give up it's national currency, and all the political privileges that go with it. Was it an experimental whim doomed to failure, or are their deeper rationales at play?
Why join a currency union?
On the surface, the primary reason to join a currency union is to facilitate better and more efficient trading relationships. Trading contracts within a currency zone are simply simpler. They do not need to consider, exchange rates, relative interest rates, hedging, etc.
Under the surface, a damn good reason to join a currency union is because your currency is about to collapse. This reason alone will cause the Euro to grow over time. It is the same reason that the UK will soon join the Euro, and somewhat related to why Greece will never leave the Euro.
Beyond political reasons, I cannot think of one reason why a country would not want to join a currency union.