Apart from the pleasing look of pure carbon twill in comparison to carbon kevlar twill of the same weight, why choose carbon instead over the stronger hybrid version? say in a car panel.

I'm not an expert on any of this, as my role in the business was administrative, but I do know there are two primary resin systems used in composite armor. 

One uses thermoset resins, the other uses thermoform resins, and materials like Kevlar, Spectra, and Polystrand primarily use thermoform resins allowing the consolidation to remain flexible as it catches the spall.

Thermoset resins are used in spall liners as well and the one I'm most familiar with is called the HJ1 System, which is licensed by AGY.  It is a consolidation of S2-Glass using uria-formaldehyde resins and the armor is and very hard.