Well... Seniority is something, but not everything. Seniority will help make a bad job more tolerable as the really bad schedules, work rules abuses, and bad pay go to someone junior to you. After you get enough seniority, it handcuffs you to a crappy company. Your seniority keeps your pay a quality of life better than someone starting over, but you get to watch your peers hired at the same time at other companies get paid more with more days off.

Seniority is important, but choosing a good company or career path is more important.