One can arrange one's own Legal Insurance at a fraction of the cost of BALPA.
Plus you're conscience is clear if you are of the opinion that BALPA don't deserve your membership.
Plus, if the above is your opinion, BALPA may get that message, provided enough pilots decide not to join. Maybe BALPA wil then improve their act to be more in line with your required standards.
Unless you're with BA, in which case BALPA membership is, some would say, a no-brainer. Which is a big wodge of subscription cash, so why, some would argue, should BALPA bother with any of the industry outside of BA ?