The battle between Frontier Airlines and JetBlue is about a lot more than just which carrier will get to acquire Spirit Airlines. The winner of this battle over Spirit will become a credible fifth U.S. major airline capable of competing against the "Big Four" — American Airlines, Delta Air Lines, United Airlines and Southwest Airlines. You might see JetBlue add seats to its planes as it removes seats from Spirit; its planes may end up with a fleet that has a dense, Spirit-like seating arrangement in the back and a more traditional, legroom-friendly JetBlue arrangement in the front.