True. Over the years, some airlines have emerged from Chapter 11, and some airlines have folded entirely. America West emerged from Chapter 11, and Continental did so twice. US Airways emerged from Chapter 11, only to plunge back in. But do you remember Eastern Airlines, either incarnation of Midway Airlines, either incarnation of Braniff, and the original PanAm? So, it's worth checking out an airline's financial health before buying tickets, especially if those tickets are for many month later.
I think United will be fine in the long run, but I'm less confident in the future of US Airways. I would buy US Airways tickets for the next couple months, but I would hold off on any US Airways tickets for this summer.
The subject of this thread, ATA, is cutting way back at Chicago-Midway. ATA hopes to regain financial viability by concentrating on Indianapolis.