I agree a roll cage is not going to prevent a roll over, but once you have one it is much better to have the cage, however failing that, my advice is to
always wear a seat belt even off road, as two Irish electricians on my 1st construction site job rolled their Ford Escort after leaving a pub, and one wasn't wearing a seat belt so fell head first, and fatally broke his neck. I have only seen the aftermath of one rollover, a friend and his dad rolled a Ford Fairlane outside our house, after clipping a large rock on the apex of a bend, and we helped the firemen roll it back on it's wheels. I'm not sure if the new Defender has an inclinometer/warning which would be useful in preventing a roll over.
Actually looks like inclinometers with warnings can be bought
Integrating safety sensors (roadsonline.com.au)