Leading article: The pluses – and minuses – of the big tent

Alan Milburn's decision to accept an invitation to work for the coalition brings to three the number of prominent former Labour figures who have, in a sense, crossed the floor. He follows Frank Field and John Hutton in agreeing to place his expertise at the service of the Government. And there could be a fourth if it turns out that David Blunkett is also in line for a role. Mr Milburn, who produced a report on social mobility for the last government, will advise on improving the chances of the least well off; he will not be paid.

Whether or not these jobs come with pay, the coalition's successful courting of former ministers has met a predictably dusty reception in some Labour quarters. Alan Milburn, though, as a leading Blairite, is probably the greatest catch. Frank Field had long been written off as something of a maverick, while John Hutton's appointment to head a commission on public-sector pensions was greeted with more disappointment than anger within Labour ranks. Although Mr Milburn left Parliament at the last election, his defection yesterday drew open expressions of bitterness. Andy Burnham, the former Health Secretary and a left-leaning candidate for the Labour leadership, pithily accused him of "putting his own social mobility above the people he used to represent".

The interest – or, to be more accurate, the self-interest – of the Government in recruiting former Labour ministers is twofold, and the first aspect is rather more noble than the second. By co-opting these individuals, the coalition acquires their specialist knowledge and experience. It means that new ministers will not have to duplicate work already done, and that their perspective will be widened. Gordon Brown hoped for something similar when he tried to form a "Government of All the Talents", but he largely failed because the specialists he appointed were given ministerial posts without displaying any real aptitude for party politics. The coalition's recruits remain essentially outsiders, which could work better for all concerned.

Above all, though, these appointments will give the Government additional political cover for unpopular decisions – even more cover than it already has as a coalition of Conservatives and Liberal Democrats. This should assist the coalition as a whole, but it could be of particular benefit to Nick Clegg, who continues to face sharp criticism from his own supporters for even countenancing, still less joining, an alliance with the Conservatives. The more politicians from the left can be found to argue that, say, curbs on housing benefit or tax credits or student places are unavoidable, not just for financial, but for social equity reasons, the more difficult will be Labour's job in opposition.

Those very benefits for the coalition, however, could carry drawbacks for politics as a whole. The more erstwhile opponents are harnessed to the Government's cause, the less it looks like a two-party coalition and the more it resembles a government of national unity or consensus. At a time of great national crisis, that might be a desirable option: encouraging all to pull together for the national good. But while the economy, and specifically the budget deficit, undoubtedly calls for severe remedial measures, we are probably not facing the sort of national crisis that would warrant an all-embracing unity government.

There are risks in not having a strong opposition in Parliament that were vividly illustrated by the lack of serious challenge to Tony Blair's arguments for the Iraq war. A one-party or even two-party government with a clear working majority is one thing; a government that makes its tent so big that any objections are to be found only on the outermost fringes is another. The consensus must never be so broad that genuine and reasonable alternative voices are drowned out.