Might be cognitive Bias, and not the most trustworthy of sources, but i'm writing a mail to Schroth as we speak:
https://www.pistonheads.com/gassing/topic.asp?h=0&f=18&t=1273077&i=20
"Interesting thread as I took it upon myself to ask Schroth themselves whilst at the Autosport International earlier this year on the subject of harnesses in cars without a cage . Schroth told me that in their tests those fitted with harnesses fared better than those fitted without (3 point seatbelt), cage or no cage. Through tests and research they stated that most injuries during crashes on track are caused by the person going out through the drivers window (which a 3 point seat belt will push you towards on impact), hitting ones head on the roof once the car has turned on its roof (not crushing, but falling out of the seat with 3 point belts), and impacts from the side, pushing the person out of the 3 point belt (they are designed for front impacts and are pretty useless for side impacts). A proper harness in these situations is much safer.
The assumption that a harness may keep you in one place and not allow the person to move away from a crushing roof whilst perhaps plausible in theory it does not take into account the many other types of incidents that occur where the harness can prevent injury. My thoughts are that in such a crash a 3 point seat belt is going to over little to no protection, can you really move out of the way? and the only real solution is to fit a cage, which of course brings us back to the original argument.
My opinion on the matter is that a 3 point seat belt offers the least protection, support. Next up, fitting a harness can prevent injury in more cases than the 3 point belt, it also provides excellent support but should really only be used with (FIA approved) bucket seats. Fitting a full cage offers the best protection but I would argue that in anything but a dedicated track car it is a step too far for most."