Why should the Anglo Welsh UK allow a break off Scotland to keep the pound?

Question

How do we know that Salmond's Scotland wouldn't be like Greece? spendthrift and into unrealistic public borrowing, thus weakening our currency.

I think that upon their independence, Scotland should join the Euro currency they so love. And the sooner they get their independence the better! The last thing the rest of the UK needs is those snakes in the grass stabbing us in the back at every opportunity.