what? The entire point of building a solid rig is to keep it rigid. You don't want moving parts twisting and changing while the suspension moves. The idea that you want the left side moving differently than the right side is just plain wrong. Please show me, relevant to the topic aka chassis, where you want to have slack between the different sides of the chassis. The only somewhat relevant argument to that would be heavy haul rigs where the torque load is better served in a semi torsionable frame to prevent the frame from bending and staying.
In a stock application the slight twist in the frame is acceptable however the vehicles being discussed are not applicable to the factory mentality. Hanging off more weight, more travel, more rotation and torsional load means that things need to be adjusted for that. Increasing twist and load on the frame only fatigues and creates the ability to weaken the material.
ETA:
I am not saying only go upand run the angled piece, I am saying run the straight bar and also tie in the factory x brace. I am not sure if you are thinking I mean only run an angled/bent bar, and thats where your statement comes from? But I am saying, run a var across, but tie in to the subframe there, be it an addition bent bar, or however it is intended to be made/installed.