in this context it is the fear of the lash, that would force the soldiers to turn up in whatever ordered dress, in case the side arm would not do - Wellington would order to carry muskets, additional he commands roll calls for every our.
It could be - that under arms - different disciplinary measures could be taken - instead when not being under arms.

For me it just looks like to give and order and force soldiers to obey it, they have to appear in a certain dress at roll call, so whoever is absent - can be easily identified.

In this context, in my view, it has nothing to do with the honour to wear side arms, it would be a sign of punishment to take them away for an individual or a regiment, especially the case when the unit did wear sabres additionally to the bayonet.