Because using a hardened bolt around other harden bolts is stupid? Makes you wonder if anyone used the softer bolts in suspension and died. Now that’s smart. Kind of reminds me of the twos springs in the M-60 bolt. They looked alike, one was capable of handling heat. Of course, one was cheaper. So, they used both. Clean the gun and the springs looked identical. Fire it and you had a 50-50 chance of failure when the springs got hot.
Yep, stupid as hell. Saving a few cents is not always the smart way to go. Keep hardened bolts around suspension, not a bad idea.
Ah....NO! This is no where near the suspension. And you don't use hardened bolts all over the place to make something fool-proof. In fact you can cause problems by using hardened nuts everywhere. It sounds like a good idea but it a really bad one. You see when you get old you will realize when things are made fool-proof they just make better fools!
My picture is a 10.9 factory bolt holding a non-critical fastener exhaust hanger and the nut on the other side not even a nut! It is a tin "nut plate clip" good for about 15ftlbs! That would never take 10.9 bolt torque nor should 10.9 torque be used here.
You see what is stupid and dangerous is spec'ing out improper fasteners for the job. A 12mm 10.9 on average takes 135 Nm or 100ftbls. A smart mechanic knows by bolt rating what the torque should be. Then you want to be exact you look at the manual and see if it jives. You can have a misprint in a book. We see that all the time especially when translated from different language or converted from metric to SAE. But a 10.9 is a 10.9. So lets say you have a semi captured ball joint in an "A" arm that would be held with a pair of M8 8.8 bolts normally about 20NM. The factory puts in a pair of M8 10.9's. Mechanic then torques the bolts to 50NM damages threads in ball joint and does not realize it. You are going 160MPH at Autoclub speedway and the ball joint blows out you die. There is a whole science to fasteners and Ford has chosen to ignore it on the S550 mustang. Use of prevailing torque fasteners in non-critical areas are but another unique Ford thing.
Proper fastener use actually keeps us safe especially if you are "on track" and a DIY guy. The track environment is really harsh. I started with a new 2004 Z06 vette and made it into a racecar. By the time I sold the chassis it had been through 4 motors and only had 17k miles on the chassis but the car felt like it had gone 300k miles. I sold the car to a friend who wanted to continue to race it. He knew why I was selling. There are so many parts to break it is impossible to keep up even when targeting the high percentage failure points. That's why I race a new 2019 mustang. Our environment is brutal. Sure enough the most rediculous thing failed on him the 2nd race he owned my old car. The harmonic balancer grenaded. There is no way to predict that.