As chris says, the problem area is the dogleg, also the way the tubes are bolted togeather means they are more likely to pivot in relation to each other in an accident allowing the cage to collapse.
Thing is, not everyone wants to go to the extent of having a full multipoint cage. its extremely expensive, means chopping your interior trim up, or just removing it entirely. Usually means no passengers in the back, etc etc
A friend of mine fitted a simple bolt in rear cage in his classic mini. It saved his life when one of the rear shocks locked solid. The resultant loss of grip span the car and he ended up in a flooded ditch, upside down. The "crap" cage prevented the roof coming down and wedging the doors closed. Without it im certain he'd have been unable to get out.
So although it may not be the ultimate in rollcages a bolt in one CAN save your life.
Oh and I like the contrast of a white cage in a black car!