Some people here imagine that a seniority system protects them from being abused by management, however I'm sure we all can think of a number of fellow pilots who have still been screwed over in other ways despite the existance of secure seniority lists, cr@p simm/check rides being only one example of how easy this is to happen.
And, by the way, I do agree with the fact that a seniority system doesn't always protect you. Fortunately, a decent union can protect you against the nimrods who occasionally find their ways into a position of power.