I heard of a guy who had a full cage and harnesses in his Evo, got t boned at a junction at low speed but his head hit the roll cage causing catastrophic brain injury. If i had a roll cage I would wear a helmet , but roll cages and harnesses are generally a pain for road cars imo
I had a harness bar and harnesses in my Evo and they were a real nuisance on the road but agree with Ekona , have a bad enough accident to roll over and cant see you getting any more injured with harnesses than without (As long as they are fitted correctly, most arent! )