When the Klingon leader General Martok (who was recently elected to public office in New York) wishes you a merry Christmas, you should at least try to respond in his own language. There's a limit to the Yuletide cheer that the universal translator can express.
Here is Mashable's handy guide to saying "merry Christmas" in 40 languages, including Farsi, Lithuanian and Korean.
It's more an indifference. I don't mind other people enjoying it or anything. If a cashier for instance would tell me "Merry Christmas", I'd still give them a courteous smile ;)