I agree 100% with @
Stotty. @
praiano63 uses it more often than I do and he produces some of the best tunes on this site. Maybe check out some of the cars where he used it and which ones he didn't. There might be some insights there. I think that he likes using it so if there is a car in his garage without it, you know that it has been tested and that you should never apply it to that car.
Those are the cars that i´ve tuned with 'Improve Body Rigidity' .
I don´t think they need absolutely this tuning option, but during the process , i have found some stability problems on most of them and install it.
You can´t take it out and it change the behaviors and balance of the car. So, you must install it to use my tune on this cars.
I´m sure you can have a good result without it too but the tune must be very different. To resume, 'Improve Body Rigidity' can be used to reduce an excess of oversteer and/or if the car swing a lot on his line over springs and dampers.
Ferrari Enzo FXX
Pagani Huayra
Ford Shelby GT500 15th Anniv
BMW M3 CSL
Ferrari GTO
Chaparral 2J Special Chris Tune'
McLaren F1 Stealth Model
McLaren F1 GTR - Kokusai UK
Ford GT LM Spec II Test Car
Renault Sport Megane Trophy
Pescarolo C60 Hybride Judd
Peugeot 905B EVO1 1Bis LM
Mazda 787B Stealth Model
Mercedes Sauber C9
Audi R18 TDI 15th Anni.
TVR Speed 12
Mazda Furai
Shelby Series One Super Charged
Shelby Cobra 427
BMW M5
Alfa Romeo 8C Competizione