In my experience BALPA are very weak. Seems to be a different story with regard to BA, but when facing up to other airlines......well, they don't do too good a job IMHO.
Legal Expenses Insurance is much cheaper and, from my own experience once again, they're more willing/more likely to offer legal support than BALPA. I've seen this occur in other cases.
Not to mention the horrendous membership fee.
My vote ----- out!