Currency sharing makes perfect sense, for a while at least, for businesses on one side of the border who buy and sell with businesses on the other. The SNP have been particularly careful not to go for the "big bang" approach (separate currency, true republic etc) as such fundamental change really would have scared more people into voting no than it would have encouraged people to vote yes.
However, assuming independence were to come, I'm certain that currency separation would happen in only a few years, and a referendum on becoming a republic in about a generation's time.