Melmoth,
a joint currency union - where the currency is seen by investors as the sum of both nations - is seen by those in the (r)UK as rather like having a joint bank account or joint credit cards with your ex-wife.
would you have a joint bank account with your ex-wife?
we (and i lived in Scotland for a decade, i got married in Scotland, my eldest was born in Scotland and still lives there) would not want a currency union with an independant Scotland for exactly the same reasons we do not wish to join the Euro - because we would be effected by, and tied to, something we have no control over.
this is not some anti-Scottish plot, its the realisation - perhaps not really grasped by some on the 'yes' side - that if Scotland decides to become independant it will be a foreign country, like Ireland or France, or Sweden, and will be treated as such. You can't be independant for one convenient thing and then decide to be part of the gang when dealing with something else.
if Scotland decides 'yes' then i wish it all the luck in the world - as you can imagine, i have a selfish interest in Scotland being a stable, prosperous country with excellent relations with the (r)UK - but not at my expense.