Wander, can you give a short list of what they have achieved? I know they worked hard for the post-service widows pension. Lobbying for those who are still serving, and not members of the society, should not be one of their roles.
A case study they sent me to encourage me to join refered to a Brigadier who was disadvantaged because of retirement dates and inflation. I wrote pointing out the the brigadier had a rather larger pension than I did. I had a very nice letter from a Maj-Gen conceding my point. I did not join and do not know what they could do for me.
My opinion there is the same as of trades unions. They should not be concerned of amended T&Cs for people who have yet to join the union, but they do. They do, I suspect, to ensure that these new members are union-orientated from day one.