A reduction in overall capacity would benefit airlines like United Airlines.Shares in United Airlines (UAL 9.52%) had soared by 9.4% as of 1 p.m. ET today. It may seem odd that shares in network carriers like United are soaring after another airline, Spirit Airlines, released an SEC filing warning that it has doubts about being able to stay in business, but that's probably the reason for the surge in United and others like Delta and American Airlines.What Spirit said and why it matters to United AirlinesDig ...