The independence campaign has to hand-wave the issue, and can't outright declare that Scotland would almost certainly have a currency-union with the UK, because it's ultimately contingent on the UK government.

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.