Why PRS is a must in Immunohistochemistry?

Since NO IHC staining machine has ever been properly calibrated for 80 years, we must need an on-slide control on each and single IHC staining. 

In addition to the reason above, we should also need a scale as a quality indicator in order to comply with ISO 15189 for each IHC staining, which is considered a semi-quantitative test.

A map without a scale can never be a qualified map, neither does an IHC stained slide without a density scale.