I have noticed when reading articles on building planes dry brushing doesn't seem to be used. I am very used to this, being an armour modeler. Is there any reason why people wouldn't DB?

I think one reason could be that there are very few areas of raised detail on aircraft compared to armoured vehicles, most detail is recessed. However I do sometimes drybrush small areas to simulate well used walkways and round the edges of intakes. Also sometimes to show old camouflage showing through new on wingtips etc.

Most DBing on planes occur on the cockpit where most of the raised detail is. Martin is also correct about some of the external areas that receive DBing.