Actually, I'm sure we've all done this sort of thing a few times: the phone rings in a situation where it would be completely inappropriate to answer the phone. But this particular call is of such startling importance that the call must be answered, just this one time. Because the call is that important, and can't wait.
The widespread use of cell phones is still in its infancy. I hope that one day it will be considered very rude to loudly speak on a cell phone in public. To have your ringer on any setting other than vibrate. To answer during movies or meals. Just as it is now to fart or smoke in close proximity to others. Do people do it? Yes. Is it common? No. Is it legislated? In the case of smoking, yes!
Air travel is generally an uncomfortable experience in the best of cases. Cell phone use is disallowed because of the additional discomfort that would be incurred to most people. Unfortunately, until some social norms have been established, this won't change. That will take time. And in the meantime, it will stay regulated.