As mentioned the strongest structure effectiveness is triangulation.With a back halved fullbody car with a high main hoop an x-brace will minimize any lateral flex.With funny car style cages in full tube chassis you have more bars and lower mainhoops and can eliminate the x-brace.With a basic stock floor pan car with stock front suspension you don't necessarly need the X-brace but it does help especially if you eliminate the back seat and trunk floor pans and replace with aluminium..