Other forms of mass transit often receive government support because the government generally has more say in how they operate. Local transit systems, for example, receive financial support to help keep fares lower. The public benefit is that the poor are more easily able to afford the cost of transit and the burden on the public highways is reduced during peak travel hours. Amtrak receives a federal subsidy but is also forced to maintain a large number of unprofitable long-distance services either by legislative fiat or as payback for support from individual members of Congress. Were Amtrak to shrink to the Northeast Corridor and some of its California operations while ditching everything else, it would be profitable. Airlines receive EAS funding to maintain service to small communities which can't support commercial air service on their own.
I'd be willing to support limited government subsidy of the airline industry given some conditions; for example, capping fares at 15 cents/mile for routes over 1000 miles and 25 cents/mile for shorter routes. But I also don't see the pressing need for government subsidy of airlines when there are clearly carriers which are able to operate profitably in the current economic environment. If United or US Airways or whomever were to go out of business, other airlines WOULD eventually pick up the slack, gaining the pricing power to become profitable and adding service (and emploment) to accomodate demand. The very high cost is the number of jobs lost at a failed airline, but perpetuating a failed business model isn't an efficient use of the nation's resources.
Just opening the public purse to United, without a business plan which is viable in the long term, isn't the answer though. What next if the airline burns through most of that additional $2 billion by this time next year? If you're an employee, ask yourself if you'd be willing to lend UAL $25,000 in cash (in exchange for some ownership stake, $25,000 x 80,000 employees = $2 billion) out of your own pocket with the way the business looks right now.