Cameron's first problem is that he actually believes it, the Union he is waffling on about is long dead, the state is ceasing to serve or deliver for large numbers of people in Wales, as far as I can seen Scotland politics is now fundamentally different as are Scotland priorities and perhaps the Scottish peoples perception of their place in Europe and a wider world. The harsh reality is that behind Cameron's sentimental pitch lies a desperate need for Scotland's oil - the Conservatives wasted millions (if not billions) in the 1980's funding tax cuts for the rich and subsidising the disposal of state assets to their friends in the City.
The pre Thatcher Labour Government did nothing to establish a wealth fund in which to store future anticipated tax revenues from the North Sea (as was the case in Norway and may soon be the case in Scotland). The Conservatives under Thatcher and Major just blew the money looking after their own vested interests, New Labour under Blair and Brown did nothing to change the situation and opportunities were wasted. Given a logical non emotive rational debate, freely reported in the media (and here I include the BBC which no doubt will struggle to hide its bias)then we cold be in for an interesting debate.
Cameron's second problem is that we no longer live in the 17th and 18th centuries. The once perceived political necessities of curbing the divine right of kings, protecting Imperial trade, controlling Ireland and Scotland for historic strategic reasons and conceding the City (and its interests) a free hand in world trade are all history. The Empire is long gone, and any real perception (outside of the pages of the Daily Telegraph, Daily Mail, Express and the Sun and some parts of the Conservative Party and UKIP) that the UK remains a real power and influence in the world died quietly at Suez in 1956.
In Cameron's remarks I do detect a combination of arrogance, ignorance and a lack of understanding of how the Union has actually worked and a basic understanding of how states actually work. Over time, peoples have combined, been conquered, formed, reformed, participated in and benefited from the existence of states, many of whom have been multi ethnic in nature. During the same period peoples have left failing states, established new states and re-established old states. It's actually part of the Human condition that's what we do.
The Union (as is) is no longer working, it is not delivering basic economic opportunities to people as individuals or as communities across significant parts of Wales (and Scotland). So simply appealing to emotive sentiment and arose tinted vision of what is effectively a long dead vision of the past won't do and just won't work. None of us can live of nostalgia, it won't butter any bread let alone buy any. I think at we are in for an interesting few years as the debate on Scotland's future develops. Personally I think that the Scottish Government should call in international observers for the referendum to ensure that poll is a free and fair vote and any Unionist skulduggery or interference in the democratic process can be avoided.