Well in the course of my career so far I have operated in two airlines who have NOT had a seniority list aswell as in one legacy carrier where seniority was king. I would have to say my preference is to have some sort of seniority as it gives a structure to the postings and promotions. With no list, everything is very 'murky' when it comes to who is awarded in command vacancies, training places, leave etc... There is always a suspicion that, far from things being awarded on merit, they are awarded on if your used to fly with the chief pilot in a previous life, or if you get on with the guy who interviews you for the new position. I have always done well out of this and twice been offered commands before more 'senior' colleagues. It is just not ideal, I liked the security of knowing when I was going to be eligible for command, even though at the time I was 10 - 12 years away from one. I liked the fact that everyone was given a 'crack' at the command course regardless of people thinking they were 'toss*rs'. Remember that the training department always had ways of ensuring those not suitable received re-education.
The only problem I can see with seniority is it does restrict your ability to move freely between airlines when you attain a certain level, perhaps this is a good thing as flying is one of those jobs when 'stability' in the workforce is important for 'safety' reasons. And at the end of the day I think I should get a better seat on staff travel if I have been in the airline longer than the person in front of me in the que, likewise I remember once I missed out on a first class seat once as I was trumped by a check in staff member who had been in the airline for 24 years as opposed to my 7 years, once again this is more than fair in my opinion.