A good seniority system guarantees the MOST important thing in aviation: SAFETY.
No contests of who does the trip with the least fuel, who arrives fastest, etc just to gain the boss' attention to get promoted. Everbody gets a chance when it's time.
Living and working in central Europe, our seniority system is mainly limited to your progress from right to left. Other things, such as vacation, scheduling, transition to other types are not purely based on seniority. Requests for certain flights are limited, with everybody, even the most junior FO getting a reasonably fair chance. As pay doesn't change for the aircraft types, it being based on years of service, the company sets up a training schedule based on economic needs with request / wishes being fulfilled based on seniority