As said - rubbish. The insurance companies normally judge how bad the accident was and reduce the NCB accordingly.
IE. if you had 5 years NCB and you scraped someones car in a car park, they wouldn't remove all of your NCB.
However if you wrote your car off along with 3 others they're likely to take all of your NCB.
Really - most insurers simply remove 2 years NCB per claim.