Performance evaluations should not suffer because of personality.
Why not? If you let a personality conflict interfere with how you do your job, then your performance suffers in the eyes of the person doing the evaluation. Bluntly, either you find a way of meeting your boss's expectations or you start looking another job.
No offense intended, but I don't think most pilots, flight attendants, clerks and most mechanics get that aspect of being in management, especially if they've never worked anywhere else but AA or another large company. They're for the most part interchangeable with anyone else with the same job title, and simply have more options for avoiding personality conflicts.
When you've got a couple clerks or pilots who don't get along, they tend to bid away from each other and avoid conflict. The last thing anyone wants is a pissing match in the cockpit at 35,000 ft, a catfight in the galley, or a boxing match in the belly of an MD80.
The fact that managers are held to a slightly higher standard, and are expected to work thru differences like that, shouldn't be all that shocking.
Something else that most people outside of management don't ever see... Performance evals are done online. I sign my employee's reviews first. They sign it second, and have a place to put in their comments. If they disagree with my assessment, they can write down their reasoning or opinion for the whole world to see, and I (rightfully) can't do a damn thing about it. If I knock them down on their next eval because of what they said on the previous eval, it's now retribution, which puts my job at risk.
So, while there's some degree of subjectiveness to the eval process, it's also mitigated somewhat by the way it gets recorded.