But it does not have to be a write off mate. And say you do cause the crash (through a simple error or likewise) or the blame cannot be pinned on anyone or something, if the 3rd parties insurance will not cover you, the bill could go into thousands to get the car fixed.
I was lucky with my Weber that the 3rd party admitted all liability when I had a crash, otherwise the car would not have been covered and I would not have been paid out.
Also, don't forget that they dont have to be a write off, they can cause a crash which can leave your car a write off, but still get away.
If your going to drive a car worth thoudands, I think your best going fully comp, just to be safe and also put your mind at rest.