I have to say I don't really understand why it would be in the UK's interests to form a currency union with a separate nation, act as their lender of last resort etc. I also don't understand how it would be sensible to do this unless a common fiscal policy were in place across both nations. And if you have a common fiscal policy, what's wrong with the current situation?
Equally on the 'no assets, no debt' argument: obviously if Scotland were to leave the Union, they would take a share of the Bank of England's reserves. In that sense they would absolutely get some 'assets'. This is a totally separate issue from whether or not (or on what terms) they get to keep sterling as a currency. Sterling is not an 'asset' in and of itself, it's a currency in which assets can be counted. No-one's quibbling that Scotland would get its share of the Bank of England's assets, or even that they can keep those assets in sterling if they want to - just whether they will get any say in the management of sterling - the currency - after independence (they won't).
They are basically saying that if they can't carry on having a say in how sterling is managed, once they've left the union for which sterling is the currency, they will default on their debts. This seems unwise to me, as it will mean - having abandoned the BoE as lender of last resort - that they then make themselves a credit pariah around the world and thus push interest rates on any subsequent debts sky-high. And natural resources or no, all governments need to borrow.
It's a bit like saying 'OK, well if you're going to ask me to stop turning up and trying to have dinner at this member's club I'm no longer a member of, I'm going to trash the place and cut up all my credit cards and get in trouble with the police, so nyer." And then looking surprised when their application to join other clubs is turned down. It doesn't make sense.
Oh and FWIW I don't really care whether Scotland leaves or not. In some ways I'd like to see it happen, just because I'm curious to know what would happen. I'm just morbidly fascinated by the debate, as so much of it seems just a bit nuts 