Those shapes are there in the earliest concept drawings of the shuttle - that is, once TPTB realized they could not afford a shape other than simple shoebox. Probably a sort of "we can't do wings on this budget but these are their futuristic equivalent" thing.
It isn't completely unrealistic that such curving would have a beneficial aerodynamic effect. I mean, in reality this specific type of curvature probably won't, but curves like that do reduce power-wasting vortices in modern airfoils. Add a bit of artistic license and there you have it: a flying brick made more stable and fuel-efficient in atmospheres because the curves redirect the airflow at the all-important corners.