Interesting that Jeff at SNP Tactical Voting has written a piece about who will become the Scottish Parliament's Presiding Officer next time round, when it's Labour's 'turn' for the job (following spells in the chair for the Lib Dems' David Steel, the SNP's George Reid and the Conservatives' Alex Fergusson respectively). I think this is based on two false premises - firstly, it's far from clear whether Fergusson will follow his predecessors in being a one-term Presiding Officer (unless there's been a clear-cut announcement I missed somewhere along the line). But the other point is that if the arithmetic is again tight after the next Holyrood election, Labour is very unlikely to remotely care whether it's nominally their 'turn' or not - they won't want to sacrifice one of their MSPs, and simply won't put someone up for the job.
Even in 2003, when Labour won 50 seats to the SNP's 27, there were some doubts about the wisdom of the SNP putting George Reid forward. After all, the overall majority of the Labour/Liberal Democrat coalition was just five seats, and Reid's election bumped it up to six. If the Presiding Officer had instead been drawn from the coalition ranks, the majority would have been four, and the SNP as principal opposition party could have looked forward to a slightly enhanced chance of inflicting the odd defeat on the administration over the next four years. As it was, the fact that Reid was so obviously the right candidate at the right time, and that he clearly wanted it badly, and that the SNP probably fancied the prestige of having one of their number fill such an important post, meant that his candidacy had a sort of inexorable logic to it, whatever the arithmetical downsides.
But of course Jeff's right in drawing attention to how crucial the choice of PO can be. If Alex Fergusson had still been a Conservative MSP on Wednesday, he would have voted in favour of the budget. As it was, he couldn't vote at all initially, and then according to parliamentary convention had to use his casting-vote to vote against. A Liberal Democrat or Green Presiding Officer would by contrast have seen the SNP's budget pass by two clear votes.