Agreed, after watching the last debate I was stunned by the extent to which Salmond tried to handwave this.
If they did, the unionists/UK government could attempt to call their bluff by having UK politicians declare that a currency-union would never happen. It's doesn't cost the unionist politicians very much credibility and it undermines the independence campaign on that key issue, whether or not a currency union is a strong likelihood in the case of independence.
The thing is though that in a currency union there's only so far the two economies can diverge before they have to adopt different currencies to maintain stability (Greece and the Euro is a fine example of the bad end of this stick) - which kind of defeats the purpose of being independent, to an extent.