Gimmick, really. Think of it as an additional voluntary excess, which has much the same effect. Mind you, even heard of a company allowing one claim a year without impacting the NCD, but they were more expensive in the first place. Don't think they will be around too long!
Its not so much the NCD that matters as being able to answer 'no' to the right questions, a couple of claims in 5 or even 10 yrs can be a problem. Look at the bottom line cost and the cover. Particularly the exclusions!
Personally I will take a big excess to avoid paperwork for a minor claim, rather than claim for a little scratch, get it fixed, move on.
Been happy with Fastnet in southampton.
N & G have just given me 10 % introductory discount and NCD even though I haven't had a policy for 20 years. Even before knocking that off, their quote (small boat policy) was by far the best.