Benny,
Agreed this would in theory produce a roll neutral setup, whilst going straight. But these cars are far from ideal stock and given the spring is an arch, as it compresses you will still likely end in either roll understeer or roll oversteer at some point in the corner, if that make sense. I.e the roll axis will change angle as the car rolls. The only way i know to avoid this is to use rear sliders, in lieu of shackles.
I guess the idea is to optimise an element of the corner to produce the best characteristic, potentially at the expense of the other aspects. As you know its always a compromise.

I think most people extend the rear hangers because that is the only option they have.
Rear hangers will help to ensure the angle change on the rear is minimised, thus keeping the spring at a more constant rate. Given that it stiffens/softens with changes in rear spring shackle angle.
We have a little luxury with the coupe (should the rules allow) to play with both front and rear easily. therefore playing with the front can minimise the impact of playing with the rear shackles only and may help fix an underlying issue. I guess it depends on what you are trying to fix or optimise for.