First post on the GRIP forum yay ! And sorry, i've never played any Rollcage ! So another point of view i guess ?
I'm just talking about damages in Races, mostly cosmetics. As codemonkey said, you need to see and "feel" the impact when a missile hits another car. Smoke, sparks, flames and parts flying ! Otherwise, no effect on driving. I love playing Flatout 2, and even if you rammed trees 3 times at 200/250kmh and your wheels aren't spinning straight anymore, you have no difference on driving (except when you lose 1 wheel but nah, can be frustrating).
But GRIP is no FlatOut, and i have no idea of what the UE4 is capable of. But really it would just be cosmetics.
I've seen the idea of self-repair with drones. What if in a distant futures, cars had something like "disposable armor panels" that could be dumped and replaced by drones (or something else) while racing ? Just having an idea right now. Take the moment in Death Race when they dump the Tombstone, but do it with armor panels and stuff in a more "techy" way of course.
Also, there are people that like their cars always shiny and clean, and some that don't mine scratches, bullet holes and missing parts. So this kind of instant repair could be sort of "on-demand" ?.
So far i'm enjoying this game a lot !