Earlier quoted context omitted.
"currencies that a sovereign Scotland will not control" And that's the key thing. They would be changing masters, probably for the worst. As a former Euro supporter and now Euro sceptic I would recommend them to keep their lead in their own hand.
But that doesn't seem to be what they want. I agree about being skeptical of being a small, relatively powerless country in a currency union with a much larger economy, but that's the case for them regardless of whether they're on the Pound Sterling or the Euro.
Their best option is have their own currency.