Paddy Ashdown was right to say that the Nationalist parties could never in political reality vote down a Labour-LibDem coalition. He cited the 1979 experience.
A rather more immediate reason is the Scottish Parliament election on Thursday May 5th 2011. The next Welsh national assembly elections are due at the same time.
First Minister Alex Salmond could not want to seek re-election as the man who put the Tories in to power in Westminster. However much the possibility of conflict with a Tory-led government with one Scottish seat could possibly assist the SNP in the long-term, that does not work if the SNP has to actively bring it about.
A (hypothetical and unlikely) Labour-Liberal Democrat coalition to talk constructively to other parties, including the Scottish and Welsh Nationalists, and indeed the new, inclusive, pro-cooperation Conservative Party too. Labour is already in government with Plaid Cymru. But the Nats could not hold the government to ransom, unless the Conservatives were to back them over a specific issue, or they were willing to back the Conservatives to get the government out.
However, we do face a major problem of geographical polarisation, whoever governs.
A Tory-LibDem coalition would have very few Parliamentary voices articulating the interests of the north, London, Wales or urban Scotland. A Labour-LibDem coalition would be accused of ignoring the Tory majority of English MPs (based on 40% of the English vote), and would have few Parliamentary voices from the English south and south-east.
In an era of sharp cuts in public spending, this matters. And while this partly reflects the sociology of political support in the UK, it is an issue much exacerbated by our electoral system. On 16% of the Scottish vote, the Tories would have 9 or 10 MPs in Scotland under PR. That arises both from the immediate distribution of seats to votes, and in the long-term consequence that the Tories and Labour have effectively disappeared as electable forces from large parts of the country.