Every extra detail you add to a job increases the cost.
Machining the pocket for the insert. Costs.
Pitching out, drilling and tapping the threads for the bolts. Costs.
Sourcing the brake rotor or cast iron insert costs.
Machining, drilling and countersinking the bolt holes in the cast iron insert costs.
I'm not going to do any of that unless a machined steel surface is shown to be completely unsuitable.

