Generally the NCB scale goes like this :-
1 year 30%
2 years 40%
3 years 50%
4 years 60%
5 years 65%
Each claim will result in a 2 year stepback. So, for example if you have a claim on 3 years, you lose 20% going back to 1 year. If you have 5 years though, you only lose 15%.
Now, add in the extra premium (sometimes in the region of 7.5%/10% extra) and the fact you'll only lose 15% if you claim, and take into account they can still load your premium - it starts to become a pointless pound swapping exercise with the insurer. As Muncher says - better to keep the extra in your pocket.
Also complications result when trying to transfer ncb to another insurer that's been the subject of ncb protection - you could have 5 years ncb after a claim under protection, but another insurer will only allow 3 years of that - not 5.
As I say, you've got to be paying at least £1000 a year to even consider it.