I know it all comes down to the look your trying to acheive but is there a significant difference between using a softbox and clipping a sheet of Tough Spun to a light?
A softbox is typically more controllable, no spill, can use eggcrate, and the lining of the softbox is typically a reflective material which will push more light out the front.
Nothing wrong with hanging diffusion in front of a light, but you'll invariably get better results from a decent softbox.