They haven't got one.
I have asked several Unionists why we should stay in the UK and not one has ever come up with a good reason why we should stay. They usually come up with reasons that have nothing positive to say about retaining the Union.
Scotland can't afford it: Yet we have been in surplus for years according to UK government figures and if you check out the debt the UK has it is far worse than Greece.
We wouldn't have a seat in the UN: Neither would England as the seat is currently occupied by the UK.
We couldn't afford to bail out the Scottish banks: What Scottish banks? The Bank of Scotland was taken over by the Halifax and is now called HBOS and RBS hasn't been a 'Scottish' bank for decades. The banks owners are the shareholders and not the country. It is like saying that if the company ScottishPower Ltd [HQ in Glasgow, listed on the London stock exchange and owned by Iberdrola] went bust then the Scottish taxpayer should bail it out. No, we shouldn't as it is owned by a Spanish company; so why should we bail it out? Just because a company has the name 'Scottish' in it doesn't mean that it is Scottish.
Anyway, you see the point. The Unionists cannot put forward one positive reason why we should stay in the Union.