I think the controversy over Cameron’s appointment was having an SoS (and one of the great offices of state no less) sat in the Lords rather than the Commons, though I do think on balance he was an excellent appointment.
As others have said, the SoS can provide accountability in the Commons for the junior ministers.
I think it’s all about getting the right balance and it’s never going to be perfect. People want individuals with expertise in ministerial roles. They also want people with a strong local connection as their constituency MP. It’s not always going to be possible to achieve both.
Torsten Bell (formerly head of the Resolution Foundation) is going to be such an asset to the Labour party at a Westminster level but to get him there he was parachuted slightly randomly into Swansea.
James Timpson I think is an absolutely inspired appointment for Prisons, but he may have had no desire to fulfil the wider role of an MP, or there might have been no constituency he felt competent to represent. Who knows.
Angela Rayner has lived experience of social housing which is amazing, but no particular expertise in getting social housing built. But Matthew Pennycook who is the junior minister responsible for housing has spent years shadowing the role and knows his onions - so I think that’s a brilliant pairing.