In return for the rather hefty 1% subscription (50% of which can be offset against tax?), you get rather hefty support in the guise of a negotiator, a legal status with which to negotiate, training courses for your CC reps etc. Allegedly there is also legal support and other such back-up. However, as a member you are part of a larger community and as such can use re-use the tools used by others to improve your own T's & C's.
Remember, BALPA is only as good as your colleagues. If you work with a group of self-centred, selfish gits there may be little point in joining but if you are one of a group who care about each other, then you have no reason not to join. From my own perspective, I believe BALPA have done a reasonable job at saving my pension, sorting out a basing policy and increasing my pay by seven years worth of increments. And these are just some of the highlights - I'll never be out of pocket.
But don't think for one minute that BALPA will do anything for you without your's and your CC's effort. They don't have hit men - they give you the boxing gloves and tell you how and where to hit the other guy.