For God's sake, guys, how often does it have to be said?
BALPA is a Union, a Professional Association, a legally convened body. The IPA, whatever its attributes is little more than a glorified employment agency. It has no ability whatever to intercede in contractural disputes bar their individual legal support, which I believe is quite good.
However, they are NOT a union, and joining the IPA instead of BALPA is a concious decision to weaken the power (such as it may be) of the ONLY sensible union (as opposed to the TGWU etc) you have available to you as aircrew.
The only pilot's body that the industry any recognition of is BALPA, so if you value your voice and wish it to be heard and considered there is no other organisation worth joining. OK, if you are one of those who regard BALPA as useless then join the boy scouts if that suits you, but it will be a waste of time and a poke in the eye to your colleagues who are keen to actually make a difference. So why do that?
BALPA, as with any union, is not a subscription to some magician who will wave his wand at your office and cow the accountants into submission; it is your own colleagues, voted by yourselves, who choose to spend their own time fighting for your rights. Themselves. Sure, they'll get advice from BALPA and hands on help when they really need it, and on occasion they'll get professional negotiators to help them too, but they will have to do their own homework first, themselves (yourselves).
Even so, they cannot ever impose results against an obstructive management if those results are not enforceable in law, and if what you want is not legally enforceable then in this day and age you will hae your work cut out to achieve it. Hopefully your reps will be skilled enough to persuade, but theycannot force. If it aint legally enforceable its a bit rich to blame BALPA for not enforceing it, cos it probably wasn't in their power in the firstplace.
The IPA is simply not in that business. yes, they'll give you a job placemant service and good individual legal cover, but precious little else. Use them, sure, but BALPA is your priority lifeline, there simply is no other.