IMO, you're a much safer and better driver when you aren't using your legs, knees, elbows, and abs trying to keep yourself in the seat. So these harnesses can prevent you from even getting into a rollover situation because you have much better control of the car. Let's not even mention you don't get worn out trying to brace yourself for 20 minutes straight...which makes you a more aware and even safer driver.
I've come up with a lot of excuses why I have not brought my E90 M3 to the track yet...the paint, brakes, etc. It really comes down to one reason...these harnesses. I have the quick fits in my E46 so I KNOW I'm planted in the seat. I can feel what's going on (I can feel something with my backside...not sure what it is [insert gay jokes]). I can drive 2 20 minute sessions back to back and get out of the car feeling physically fine. Versus someone else who is struggling to stay in their seat for one session.
So whatever argument as whether these harnesses will keep your head from getting crushed in a rollover is 90% pointless. Its like wondering if your car is going to spontaneously combust...it could happen but you have plenty of other things to worry about before you get to that point. So I will gladly take the risk of having my head support the car in a rollover for the many other benefits of these quick fits.
And there's no crotch strap. If you really tighten the shoulder straps, it will push you down in the seat even if the lap belt is tight. So I'm not sure why people think you won't slide down the seat at all.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red