It is accidents like this...
Mark Pombo Huge Crash - Road America - YouTube
That door bar design show what they are worth. This kia had very good Nascar style door bars and they moved in the shunt enough to let marks head strike the tire barrier. That being said had he had flat door bars its highly likely that Mark would not have survived the crash. As I understand it he broke his collar bone, his jaw, and took a big hit to the head. Any track prepped Z is going to have the ability to carry more speed then that KIA just something to think about. So when you thinking of skimping on a cage remember it doesn't take other cars on track/racing to cause a major accident like this one.