Leaving the euro: an emergency exit for the currency union?

There is no option for member countries of the eurozone to exit the union. On the one hand, this protects the common currency from speculative attacks. On the other hand, the euro crisis has shown how hard it is for the member countries of the euro zone to solve economic and fiscal conflicts constructively. Why was the euro designed as a one-way street? What would be the consequences of an exit option? And what are the alternative options to an exit?