On currency unions

In trying to justify their proposal to use Sterling in a separate Scotland, Wee Eck and his pal, the Wicked Witch of the West, like to point out that there are currently 67 countries in the world using another country's currency (actually, I'm not sure about that number, Wikipedia would suggest around 33, not including those using currencies specifically intended to be supranational, such as the Euro or the CFA Franc). But even taking those into account, how many are just de facto situations where a stable currency like the US dollar has become the norm, rather than formal monetary unions? How many are tiny states like Kiribati or Saint Lucia or the Cook Islands that have no practical alternative to using another country's money? How many are third-world countries like Niger or Mali who can't really aspire to having their own functional, stable currency? How can the SNP pretend this would be a desirable state of affairs to have?