"im sorry im confused about this statement. i understand pilots and race car drivers have the 5 pt. harnesses in their car seats but the physics is still the same re: the safety of a 5 point harness vs. just a seatbelt used in a booster."
The issue is points of failure, and the adding thereof. A child in a harness, sitting in a seat attched via the seatbelt, has two chances to go flying through the window - the car's belt can fail, or the harness can fail. I prefer to rely directly on the seatbelt and not use a harness on my 4 y.o.
OTOH (and I think this is the big lesson to take from that Kyle video), if you have your child harnessed in a seat that is LATCHed or belted and
tethered, you are actually adding another line of defense if the LATCH or belt should fail (still won't help if the harness fails). The top tether might keep the seat from being ejected, which greatly increases your child's chances of surviving the crash. While there has not been any particularly good testing on the use of tether at higher weight limits, the Car Seat Lady
has put together a chart of the available info. IMHO, continuing to use the tether after you've stopped using the lower anchors never hurts and might help.
I think that the conversations about 3-pt. vs. 5-pt. can only be usefully made in the context of integrated vs. non-integrated. Integrated 5-pts. would be the safest for everybody, all the time. But as alegna points out, seatbelts need to be replaced after crashes, there is a cost issue, there is not consumer pressure to integrate 5-pts. ---> I do not have the option of purchasing a vehicle that offers integrated 5- pt. harnesses for my kids. Once my son was of sufficient weight and height, it was a great relief to me to secure him directly with the integrated 3-pt. system my car came with. If he was the kind of kid who wiggled around and messed with his belt, he would still be in a non-integrated harness.