I was persuaded in the past to have it, even though I paid cash for the car, but after looking into it I realised 1. it's cheaper elsewhere than the dealers, 2. most motor insurance policies pay out for a new car if you write it off in the first year anyway so you only need it from year 2 and 3. it only covers written off cars (and you have no choice in the matter as to whether it's a write off or not).
I would probably only take it out in future if I had a car on a bank loan so I wouldn't have to carry on paying payments after the car is written off. I'm not sure how that affects PCP plans or hire purchase agreements as technically the car doesn't belong to you until it's settled.