Good or bad depends on where you are on the list. If you're at the top of the list, seniority is great. If you're at the bottom, it sucks.
Seniority is not what removes competition from the job market. Pay does. I'm all for a national (heck, international) seniority list, but the pay structure would have to be modified.
Thus if a 15 year United Captain chose to go to Delta, he would be at the bottom of the seniority list, but be on 15yr pay. Thus he would be properly compensated for his experience. The current nonsense of new company = yr1 pay has to stop.