When is the last time someone with racing harnesses smashed their face into the steering wheel, especially someone 6ft+?
In a race car, the roll cage is needed because the car usually gets ripped to bits when it crashes. Rolling is the prefered way to crash at speed (providing there is no gravel track/runoff long enough to stop in instead) due to the energy being absorbed from each roll, (the bit that hurts the most is the last roll back onto (hopefully) four wheels or the roof, providing the driver isn't unconcious) instead of a sudden stop from hitting the Armco or any other barrier, which can cause whiplash and internal bleeding.
Huge structural damage is common so the car needs the extra support from the rollcage, so that there are at least metal bars left to protect the driver if the roof gets ripped off or crushed.
Harnesses commonly dislocate and break shoulders and collarbones, a small sacrafice on a race track, they REDUCE the damage caused to the driver, but don't think harnesses and race seats are perfect either, with a 6 point harness and OMP bucket seat you can still hit bodyparts (arms and knees mainly) in a crash as you still get thrown forwards, just a lot less than with a regular seat belt.
Harnesses also commonly jam a lot, which is no problem on a track as marshalls/stewards carry knives to cut drivers free, but on the roads being able to get out quickly after a serious crash is very important, and getting trapped is the last thing you want.
Proper race-orientated seats reduce visability greatly, which is dangerous in itself, and could lead to a crash in the first place. If anything, get a road styled seat, such as Sparco or Cobra's cheaper models, or take seats from a roadsports car (example: Type-R/STi seats)
Personally, the only items here that i'd allow in my car are the road-styled seats, road-styled harnesses, and a support bar going behind the seat to support the harnesses (minimum angle for the harnesses to slope away from the seat is 45 degrees, less than this and it's not worth having them, optimum is 10 degrees more (sloping up 10 degrees - very rare, mainly only used in SUVs, vans and 4X4s) or 10 degrees less (sloping down) from being horizontal with the support bar (These are FIA recommendations for the 'saloon car' section - basically a 'race prepped' road car).
NOTE: Harness bars can be welded onto roof supports, but will need triangular supports above and infront of the bar, and VERY thick, generous welds.
My top tip: Keep the airbags. I'd rather sell a car than drive with no airbags, I know at track days they save many people from having to be airlifted to hospital.
Trust me, I'm a Marshall

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