I believe seniority is a good thing, and I am sure that many training Captains would be a little upset that some people think that when you name comes to the top of the waiting list you get a command. In the companies I have worked for, one with a seniority list there were some F/Os who were bypassed as not suitable or did assessments and failed. The standards for Command were no lower than any other airline and you only got a command if you were up for it. What it meant was that the company didn't employ DEC when Senior F/Os were capable of passing the course.
My current company doesn't have a seniority list and I find that new F/Os joining from other airlines with more "Factored Hours" AFTER me will get their chance sooner than I do. Does this mean they are more suitable for Command, possibly, but that in itself doesn't mean I am not good enough. With waiting lists for some bases very long this doesn't seem to me to be a way of creating what the company call a "Career Airline".