Ratcheting seatbelts definetly can hold tight no doubt. But if using a shoulder belt it's a bit messy since the shoulder portion is pulling the carseat up usually but the bottom part is pulling it down. The main reason I used those steel beeners is I was putting the carseat in the middle seat on an 80-series, which does not have a lap belt for the center seat. Also (the main reason I did this) was because on my son's carseat at the time there was no room for the seatbelt buckle behind his back. His seat was designed ONLY for the LATCH system, so the seatbelt went right behind his back, not in a seperate area further back. So if I used the car's lapbelt the buckle ended up right in his back, not exactly comfortable or safe. Also in the 80-series the center lapbelt really can not be gotten tight, no matter how tight you get it, when on a carseat it'll work loose in a few days. The LATCH stuff cinches down a bit better and doesn't come loose at all. I don't doubt that normal seatbelts have a very high breaking point, but the LATCH components found on carseats are NOT normal seatbelt quality, or even close. They are strong, but you only use them for kids up to like 30 lbs, then you are sposeto use a normal car seatbelt anyway.
Very true the LATCH system was created so the average joe could get a seat in correctly and tight enough, and it works quite well for that