Of course 'whinging on MN' can achieve things. How is public opinion influenced? Through many channels but it is undoubtedly true that MN can increase visibility of issues and bring about change. Lots of people don't realise that benefit claimants get such a huge discount and other advantages.
Personally I have first hand experience of MN influencing my opinion on trans issues and I know I'm not the only one. I simply wasn't aware of what was going on and was shocked when I read threads on the issue. I believe these kinds of threads raise awareness around the unfairness regarding relatively low earners that don't claim benefits and can't access these additional advantages and those that claim and can.