Seniority is not all good and not all bad, however it is based on the assumption that one airline can suit ones whole career, therefore it is something often found in career-airlines, aka legacy carriers. And as showed in the previous posts it can make it easier to defend your terms and conditions or even improve them.
But they can be a major p.i.t.a as well, for one lateral movement is pretty much impossible between seniority based companies, for another it can make mergers quite difficult. At the moment we try to integrate three seniority lists into one, with extreme differences in age and time in company between all three and it proves to be a major issue (and sadly, among other collective agreement issues, reduces my flying time to around 300 hours a year instead of 750). However seniority helped us to improve our working conditions quite a lot and even our wages.