I say Happy Holidays, not Merry Christmas.
Why? Because I want you to have a happy holiday season, no matter which holidays you celebrate, even if only New Year's.
To me, insisting on saying Merry Christmas no matter what others might celebrate, is akin to wishing someone a happy birthday or happy anniversary on your own birthday or anniversary because that's what "you" celebrate.
Why do people bitch about "Happy Holidays" anyway? Shouldn't they just be happy to receive good wishes from someone else?