The main advantage is the reduction in transactions costs and uncertainty. But there is also an anti inflation argument. One problem is that the single currency makes for a lot of inflexibility. It's hard to respond to country-specific shocks. Also, there can obviously be no domestic control of monetary or exchange rate policy, and many see this as a loss of political sovereignty.
Key issue is perfect capital mobility.
Interest rate arbitrage means this must hold. If I'd used one-month interest rates it would have been even closer. Conversely, under floating rates , if the government raises G, interest rates will tend to rise, causing a capital inflow and consequent appreciation. Thus exports will fall. Fiscal policy will crowd out exports.