from personal experience i can tell you that a seniority based system for Pilots career advancement or also the now quite common movement in the opposite direction, is like democracy, far from perfect, but beats every other system hands down. Seniority can be a stabilising factor among any group of people, the absence thereof promotes backstabbing, character assasinations and a**kissing in order to get promoted or prevent demotion or layoff.
I believe a seniority based system is beneficial to both the employer and the employee!