If the decision is made not to provide quantifiable comparisons between choices, then society as a whole suffers because it will not always get to appreciate that other, better, choices could have been made.....

I understand what you are saying. Yes a unit of comparison would be highly desireable. But since every case is different and every site has a million variables it can't be done.

Heritage choices therefore come from two sources. Attempts to allocate weightings, as suggested by EH/HE Advice notes and Inspectors' cae law - which are good though only broad brush .... and the rather less savoury miasma of vested interests and Govt. overrides for political agenda reasons.

Good luck with producing a common unit of comparison out of that lot!