Mark, the argument you cite is the least quoted. Those most quoted are - Britain being able to decide for itself rather than directed by Brussels; full control of border to limit numbers entering; trading freely with the rest of the world and, not least, the saving of the payments made to the EU! Let's not forget that it's thanks to the tax-payers of Germany and the UK that Spain built much of its modern infrastructure including its excellent AVE and motorway network!
It's never quoted because the anti EU people only quote from their point of view. Saying that Britain is directed by Brussels is the same as saying Manchester is directed by London. The parliament and administrative centre is based there. The decisions are made by the member governments.
Lets also not forget that many Spanish companies operate in the UK. Investment doesn't only come through government subsidies and even then the UK benefits from the subsidies.
The new deal that Cameron got ( We were told by Farage that to get one would be impossible due to EU law) could have major consequences to British citizens working in other EU countries if they impose the same rules on us.