Not having a currency union wouldn't stop an independent Scotland using the pound unilaterally. Montenegro isn't in the Eurozone but uses the Euro.
The Eurozone crisis has changed a lot of conceptions of what joining the Euro would entail. Basically you lose the ability to set an interest rate that suits your own country (something most of the southern Med countries have found out to their cost) and any way to devaluate your currency which can kick start an economy in the doldrums.
Once you're inside the Euro these issues are permanent so it's a huge decision to enter into it. I suspect a lot of the political class in those southern Med countries I mentioned regret not realising what they were getting their countries into in January 2002 when the Euro went live as an everyday currency for the general public.