There is a belief here that you can sugarcoat presumptuous seat swap requests with a gloss of niceties and call it polite. Politeness goes deeper than mouth service. You can't call breaking in line polite because you said sorry, excuse me in a pleasant tone of voice. You can't presume that you are not bothering somebody when you ask them to move just because you ask nicely. Having a good reason plays into whether it is considerate and polite to ask. Do you NEED to sit by your small child, or do you WANT to sit by your husband? Sometimes seat swappers really do need to ask, but mostly they don't.