Anyone who's made flat pack furniture will appreciate the contribution of a thin panel to the structural rigidity of a construct. Take a set of drawers with two uprights, a top panel and a couple of bars across the bottom at floor level. This structure will stand with the drawers removed, but any lateral force leads to rapid deformation. Until the thin hardboard back panel is tacked into place the structure can lozenge and will readily fail and collapse. I wonder if a coupe without its roof panel might experience a marked drop in rigidity, likely most important in a rollover as suggested.