Seniority lists are good for those with seniority! And the companys like them because they know that their pilots with experience won't leave. As a pilot, it's a good policy for protecting your interests, but does tend to restrict you to working for one company most of your life. The system is a bit archaic. Imagine a Barrister having to become an article clerk again just because he swaps law firms or a consultant doctor becoming a junior houseman again because he changes hospitals.
The chain is only broken when a company such as Easyjet or Emirates who don't have promotion based on seniority, or a company who starts or expands rapidly (Oasis, Jetstar) who needs DECs, starts recruiting. A situation we have now it seems!
PS. I have quite a lot of seniority, therefore I think it's a good thing!