It actually does really help the rear frame. There isn't much there above the subframe mounts to prevent twisting. More modern cars have glued front & rear windows that contributes greatly to rear structure (the M3 was the only e30 with a bonded windscreen). Cutting graceful curves and smaller openings is far stronger than big square ones too.
But if you're planning on a rear suspension bar, I'd take it out. The bar will be far more robust than that thin panel, especially if it's cross-braced to the rear frame outriggers.
I always thought that a folding rear seat is do-able. Not worthwhile, but it'd be unique...there are many cars with a similar layout that could donate the parts.