As per the subject title.
From what I can tell, it's always worth shopping around for comprehensive car insurance when renewal time comes. At the moment, I have a NCB rating 1, and some insurers offer "NCB protection", which means that they won't change my No Claim Bonus for the first claim I make in a policy year even when it was my fault/I can't identify the party at fault.
I don't think this will impact the NCB that any other insurer will offer me next year though, so why would anybody pay for this? The insurer is just going to increase your premium next year anyway, and over the last few renewal periods, I've always found jumping to a different insurer was better.