Static said it first and let me repeat it.
An airline without seniority would discourage whistle blowing, safety concious, pilots. They would have to constantly reflect on whether to speak out against new 'work pressures', for fear of hampering their chance for new opportunities within that company.
It is not in the interests of any airline to promote a First Officer who is not capable of commanding one of its aircraft, regardless of their seniority. This thread has sometimes suggested that.
There are around 5% of First Officers in the company who I work for, who will never be allowed to become a Captain, which is a meritocratic slant to our seniority based system, is it not?