I have tried freehand, and my airbrush will spray very fine lines indeed, but I would say it is worth the effort personally. I have seen very few free hand camouflage demarcations that are tight enough. Scaled up the overspray area would be a foot or two, and the models just look toy like, even in 1/48th scale. I am happy to freehand airbrush for touching up, mottling, shading and the like.
I know what you mean though - two hours masking for 30 minutes spraying!
R