We left the NSALG mostly because they seemed expensive for what they offered. Our insurance costs alone dropped by 2/3rds by leaving them. With around 120 plotholders their charging system per plotholder was a key problem.
The idea of belonging to a bigger organisation has a feel good factor and the political importance over the longer term might be useful but... The big but is on what subject would we ask and get suitable advice from such an organisation about our association. The answer is probably none.
Even in 2005 when we were established as an independent association we became NSALG members by default - probably because it helped our Parish Council feel good about the correctness of our formation. In practice, looking through the records, I see little evidence of input from NSALG and all the hard work was done between parish council officials and the committee. What was created was arguably as good as it gets, and certainly good enough.
My gut feeling is that uber-organisations such as NSALG need to justify themselves more clearly and fully - and work out a charging system that reflects real value.
(I say the above as someone who was not involved at the time of these decisions, so my opinions reflect my interpretations of what happened.)