The economy. Scotland does most of it's trading with the rest of the UK anyway so would remain affected by any decisions taken in westminster, but would have much less influence over those decisions especially in the event that they keep the same currency. Much of Scotland's economy would become contingent on volatile and finite oil revenue.
Also the UK is a single landmass , so it makes sense to defend it as such.
Also the UK is a single landmass , so it makes sense to defend it as such.