Weight? Well you've never seen a race car without a cage either have you? Don't get me wrong, I'm not opposed to seats, but it really needs to be part of a complete system. That system includes a 6 pt cage at a minimum. There are many upgrades to a basic 6pt cage so that's why I say minimum. I used to believe in 4pt roll bar, but I feel safer in a late model car with factory safety. Take an E30 or E36, then yes a roll bar is probably going to be better than nothing.
Just adding seats invalidate the "system" by directly changing how the body is handled in the car during the first AND second collision.