SAR Bloke,
When the guidance for civilian use of NVIS were formulated and JAA TGL 34 written, the decision was taken not to constrain their use by setting arbitrary limits. The weather limits were aligned to those that were specific to the type of operation - thus permitting variation according to the task.
NVIS is not considered to be a "back-up to standard night techniques"; it is accepted that appropriate techniques should be used and the TGL contains many pages of guidance to assist with their formulation. What you might be alluding to is the policy that the limits on weather for Night VFR should not be lowered for NVIS - that was jointly agreed between the FAA/JAA and RTCA/EUROCAE when the task of providing regulations was started.
We were also mindful that existing NVG use in specific phases of operation - particularly transition in landing and take-off - had been evolved by operators over a number of years; we were therefore reluctant to be specific and instead tied this to procedures contained in the Operations Manual.
We were also interested in generalising the text so that NVIS could be used in areas of aviation other than CAT; in fact the only NPA for NVIS that has been produced and circulated for comment in Europe was the ANPA for JAR-OPS 0, 2 and 4 (general aviation, corporate aviation and aerial work).
Even before the JAA started work on TGL 34, the CAA already had a limit of operations of 500' for one of their operators; that was proposed to the working group but the JAA decided, after much debate, not to align the text with that policy.
Jim