You are quite right in what you say, the IPA is not a union, and never was intended to be. It provides legal cover, an excellent employment service and other benefits that would be expected of a Pilot's association. Since the IPA was formed, many BALPA menbers have benefited as the "competition" in these fields from the IPA have obliged BALPA to improve the level of service provided in these areas to its members.
Disquiet as to the union services offered by BALPA have caused the IPA to form a separate organisation, the IPF, which has been structured in accordance with the requirements of a trade union. They have, to my knowledge, yet to have a "showdown" with an employer on behalf of their menbers. Presumably, so far none has been necessary, but one of the reasons for forming the IPF was that a number of employers voiced concerns that there was no "union" organisation with whom they could negociate - apparently negociations with BALPA were not practicable for some reason.
I notice that in another post above, BALPA's negociations with "the company" is referred to. Does this mean that, despite their vehement denial, "BALPA" really still stands for "British Airways Line Pilots Association"?