BALPA's not perfect and you'll find plenty of pilots from years ago that carry a massive chip from being let down by what really was the" BA" LPA. It has modernised somewhat and if you have a good CC and negotiating recognition then membership allows you to vote on issues that effect your own T&C's. If you are not in then you can't vote. If you are in an airline that does not recognise BALPA then it is worthwhile for the representation if you get yourself in trouble. Just remember that the Union is not there to bail out your own negligence. IALPA do much the same thing for less money, but remember that you get a reduced emebership for the 1st. year and that a proportion of the BALPA subs are tax deductible.
On balance, worth joining for the strength in numbers argument.