The suggestion in some quarters that the Scottish National Party’s Angus Robertson should not have been nominated to the Commons Intelligence and Security Committee (ISC) is deeply foolish. It might even be counted self-defeating. Nothing could be better calculated to undermine the Union than to treat its institutions like private clubs.

Is the SNP’s Westminster leader opposed, like his party, to the renewal of the Trident missile system? The fact hardly counts as a revelation. Does that render Mr Robertson a threat, in some manner, to national security? Only in the fevered imaginings of those who treat defence procurement decisions as the exclusive preserve of the governing party and, for good measure, misunderstand the SNP.

First things first. The ISC is a committee of Parliament. These days it is appointed by MPs and answerable to them. Since May, the SNP has been a significant Commons force, elected by a landslide to speak for a large body of opinion within Scotland. Parliament would be failing its democratic duties – and failing the Union – if it tried to shun Nationalist MPs and those they represent.

The point has force given the fact that the SNP has never been an abstentionist party in the manner of Sinn Fein. Peerages are shunned, but Mr Robertson’s party has long followed the parliamentary road. Its MPs might aim to end the Union and one day withdraw from Westminster, but the ambition does not make their present participation illegitimate. They have the same rights and duties as other parliamentarians.

Chief among those duties is an obligation to scrutinise government. David Cameron’s administration might have fallen into the habit of treating Trident renewal as a decision as good as made. This is far from the case. Billions of pounds have already been spent, either in design work or to prepare a fait accompli, but the Commons has yet to decide whether to proceed. Self-evidently, there is a great deal at stake. Mr Robertson’s role on the ISC can only assist the process.

It could be argued, in fact, that proper scrutiny depends far more on dissenting voices than party loyalists. Mr Robertson’s sceptical response to the killing by Reaper drone of two British adherents of Islamic State is a case in point. Ironically, in the circumstances, he first called for the re-establishment of the ISC, but his argument went to the heart of the relationship between government and opposition parties. There are questions to be answered; first, they must be asked.

What was the “imminent threat” posed by Reyaad Khan and Rahul Amin? What, if anything, can the security services say to enlighten MPs? When, and in what form, will the Government publish its legal advice? What precedent has been set for dealing with terrorist threats?

Mr Robertson will no doubt have many more questions of his own, on these and other matters. We can hope the Moray MP will pursue them with the same rigour he has shown as one of the most hard-working members of the Commons. Those who fear that his appointment marks the SNP’s arrival at the heart of the Establishment should ponder the alternatives. Nationalists suspicious of the UK’s embrace might do the same.