My Turn,
There's more to this business than just contract negotiations. Similarly, there are dozens more reasons that people chose to join ALPA than just negotiation strength.
For example, IFALPA and the regional factions (NZALPA for example) do a phenomenal amount of work in safety, providing a strong voice and expertise on a variety of matters. Left to the regulators and bean counters, the industry would be less safe than it is. More corners would be cut.
I'm not suggesting ALPA do it all but they certainly play a big part. Like any organisation of people, not everyone agrees with everything and neither should they. It's perfectly reasonable to challenge an ALPA position if you disagree with it but not in a manner that weakens the strength of the union.
Remember: ALPA is not some mythical group of people in AKL. The members ARE the union.