As a supporter of Scottish independence, even I sometimes get frustrated that the SNP don't explicitly say what their "Plan B" is (implicitly it's always seemed fairly clear - a currency union isn't the only way of keeping the pound).
Whatever you may think of Alex Salmond, he's not daft, so there had to be a reason for him consistently failing to give the clarification that obviously many people want. I would have guessed that it was something to do with maintaining the strength of his negotiation position after a "Yes". That wasn't a million miles off, but it wasn't wholly right. Here's Alex Salmond giving the clearest explanation I've seen of why the SNP are taking the position they are: