Difference with carbon/plastic composite parts is they are stiffer...at the cost of being slightly more brittle.
If a solid, stiff car is what your after then carbon parts are better. But I think that a lot of team drivers have decided that the little bit of extra flex the plain plastic parts give is an advantage in certain circumstances.
The plain plastic parts do bend a little more and deform slightly before they break, whereas the carbon composite parts will take a marginally bigger hit without deforming at all, but break straight away rather than bending a little first...which in a small number of cases might be the difference between finishing a race or not...particularly for the less gifted driver who crashes a little more often!!!
The wonders of material science!
