I suspect BALPA gets the blame unfairly when things go wrong, for example when an employer collapses, people are naturally angry and latch onto BALPA as something to lash out at.
BALPA can't perform miracles however I defy anyone to disprove that airlines with the highest membership levels have the best terms and conditions.
I've had sterling service from the association and don't work for BA. My company council has had large amounts of BALPA funds expended on it over the years. Whilst BA makes up the bulk of the membership and it rightly BALPA spends the most money on it, however it doesn't follow other companies are neglected. Quite a number of my colleagues have BALPA to thank for their ongoing gameful employment.
To any wavers, I would caution putting too much store in the words written under the cloak of anonymity. From the bottom of the page: "As these are anonymous forums the origins of the contributions may be opposite to what may be apparent. In fact the press may use it, or the unscrupulous, or sciolists*, to elicit certain reactions."