ALPA kinda makes me throw up in my mouth a little every time I hear about them. You can tell pilots aren't that smart because they put up with such a stupid system of representation. Back in the days when pilots were a valued commodity and before the days of deregulation and the aviation austerity program of the last few decades, it was OK. Now though, having independent Unions and seniority lists at every airline does nothing but pay extra and unnecessary Union management. You get no real benefit as a Union member. The contracts are crap and the Union concedes more and more while the people at Union HQ get pay raises. There are very few stable airlines and airline jobs so a central seniority number and hiring hall would be a long term benefit to both pilots and airlines. The Union could take over the training/currency program for all the pilots using a set of pooled resources that each airline has to maintain independently at this point. For all those union dues you pay, you can elect to qualify and get rated on more equipment on your time off. With some cooperation of the FAA, you could even standardize a 121 Operations Manual so that once qualified in an aircraft, you can work "casual" on that aircraft for any carrier making it much easier for carriers to compensate for seasonal fluxes in seats required on a route. Also it can reduce the time and passenger inconvenience that happens when you need to replace a crew member at the last moment. Several times I've had to wait for hours to make a flight because someone got sick or didn't show, one time we couldn't continue because the last leg went over time and the crew had "timed out" and they had to wait for another crew to get flown in from halfway across the country.
The current Union model serves no one well but the Union Leaders and the Airlines.