I have experienced FCFS and seniority boarding and hands down, I prefer seniority.
Nonrev travel is by its nature unpredictable, seniority at least gives you a hint as to your chances. Couple that with the huge commuting population at US, which, by the way, the present US management referenced when it developed the current policy at US, and it just makes sense. If you have the seniority, you can commute. If you don't, you don't.
What I don't get, is with all of the significant seniority dates at American, why they WOULDN"T want seniority boarding. They would have an AAdvantage. (Yup, pun intended)
Finally, if there's anything to angst about, it's the composition of the benefits. American's are hands down, inferior. At US, we fly for free. Period. Internationally, we pay the taxes. Thassit. If you want to upgrade, you pay for an upgrade. That is much better than paying by segment, etc. Admittedly, it would be nice to not have to pay to upgrade, but fundamentally, I would rather do that than pay every time I fly. Also, frankly if you're going to upgrade you should be prepared for it, dressed and everyone briefed to behave discreetly. If it was automatic, given the lowest common denominator, we'd probably lose it after a few months anyway.
I actually think that it benefits US for its employees to travel freely. We're the best advertisers of destinations and the benefits of travel. We're safety helpers if something goes wrong, quality control to a limited extent, admittedly not great, but stuff does filter back. . . and I flew a trip with an American non rev who hardly ever flew his own airline, it made more sense to use ZED on other carriers. What does that say about the company that you work for that you don't even use its services? It doesn't COST them anything to provide this benefit, if anything they only gain from it via ancillary revenue. Revenue that would go elsewhere if employees were just as well off using ZED as their own carrier.
I don't have a lot of hope that our policies will survive a merger, but clearly, ours is a better benefit, and given this industry, that's important.