Here's a question for you, beyond date of hire, why should a person get promoted?

Let me share with you what happened on my very last trip. I fly a large cabin business jet. Our normal crew complement is 2 pilots/1 flight mechanic/up to 5 cabin crew. Now, we took over from an inbound crew who had this full complement and ferried the plane to base as it was needed for another long range mission. We only had 1 cabin crew and 1 flying mech. To make matters worse, we received a late departure slot, plus we had to de-ice. Furthermore, we didn't have time to get the cleaning done on the ground as we always do. So what happened? During the flight back, the FA and the mech washed all the dishes, dried them off, cleaned the cabin and the galleys, restocked all items, put together a resupply list for the next crew. Did they have to do it? Nope. The FA could have simply said, sorry, I was alone so I did the best I could - not much. Mech could have zonked out in the back or watched movies. This was DEFINITELY not in his job description.
Mind you, both of these people stay the hell away from the office and off the radar.

I wrote a report to Director of Maintenance and Head of Cabin Crew about what these two did with cc to upper management, and both received appreciation letters.

Would I have said anything if they didn't do anything beyond the barebones minimum? Nope. Would the company say anything? nope. Would the following flight be more adversely affected? Yes. Was there a valid reason/excuse? Yes. Yet these two shined. Why? Perhaps because they care about their fellow crew/mission? This is the biggest problem with strict seniority system - no incentive to do anything beyond the mandated minimum, and in turn, your product won't be anywhere near what it could be. Not to say that seniority shouldn't be a factor, but end-all-be-all? I don't think so.