The parallel is valid up to a point, essentially many countries use the dollar unofficially and some officially. They essentially ride on the coat tails of the USs economic policy and as such move with it. Problems arise when the activities of the nation concerned require economic adjustments that they cannot make because they have no control over the currency, QED Ireland in the EU for example. So yes Scotland can adopt the pound but they will be at the mercy of the UKs economic policy shifting and may find themselves in difficulty with things like interest rates and inflation etc. They may also find that the general public of Scotland would prefer to deposit their money in English institutions which won’t help either.