It's not so much a question of keeping the pound, which the Scottish could decide to do unilaterally. It's just that they would be excluded from making any decisions about it. If the rest of the UK chose to, say, devalue, or to set a new interest rate, or anything else attached to that, then Scotland would be unable to do anything about it and would have no say. It would tether their economy to a weight that can be added to freely, independent of their wishes, which is not really a good thing.