The obvious and cynical reason would be to raise the profile of the issue and take it to the court of public opinion. Yes, having a high profile debate on the issue is useful. But this isn't a debate, it's an attempt at scandalizing and discrediting the issue. If the court rules in favour of NIWA, it's a complete non-story. If there's any sort of wiggle room in the ruling at all, that will become the only part publicized. Either way, this is still unlikely to have much of any impact on any scientific research or work.
The basis for the claim seems to be a thesis published by Jim Salinger in 1981. NIWA cites this thesis as being an important source for calculating the relative temperature changes throughout changes in the site of weather monitoring station. But they also say they don't use the methods described in the thesis at all. to compound this, the thesis that is publicly available is difficult to read because of the restricted access to the paper at the university where it's kept.
However, as the methods set out in the thesis aren't used by NIWA by their own assertion, discrediting the paper (as seems quite possible at this stage), wouldn't call into question the temperature data NIWA is basing the warming trend they're reporting.
As per a Parliamentary Question and Answer session on the topic, NIWA was reviewing the data in March. Since some of this data goes back to 1853, it's used internationally. So a review of the information using current methods ensures it's integrity and accuracy.
Research for this was done through Google, looking for information on Salinger's thesis and the information that follows from there. So I'm sure there's plenty more relevant information out there and I'd be happy to have that included or discussed.