They'll have a merry whatever they're celebrating, doesn't mean we can't say merry christmas. I'll be damned before I start saying happy holidays, people who celebrate others can just translate it into their festivities.
I'll greet you guys in my Native tongue then, "Maligayang Pasko!", which ofcourse literally translates to Merry Christmas!
Happy holidays? How offensive... I would just like to wish everyone to have a pleasant end to December! =p