I find it sad to think that in this day and age, we even have to stop to wonder who might take offense to simply being told to enjoy the holidays.
Around the world, for most, the holidays mean family and celebration. But to others, it's a day to take offense to everything that can even remotely be related to religion.
You are being nice and friendly when you say merry christmas and thats what matters. If you get offended at someones attempt to be friendly than you are over reacting and being too sensitive. There is nothing offensive about kindness so if someone says merry Christmas and you are offended treat it as compliment.
If you're offended by "Merry Christmas" then you're either going out of your way to play the victim or you're extremely sensitive. Now if someone knows you don't celebrate Christmas because you're Muslim, Jewish etc. and are just trying to get to you then that's another thing, but I really don't think many Christians would do that.
Offensive to whom? Who decides what is offensive? Who are you saying it to? Does it matter? There is a big difference between being inclusive and simply excluding everybody. If someone is offended, then they can say so. Then the person saying it should respect their view and not say it again to them. I don't celebrate Halloween, but I don't get offended when people say Happy Halloween to me.