I think you covered the "why" very well in your last paragraph. I opened this thread and read it, because I've always wondered why someone would not celebrate Christmas. However, with every response I feel a little nagging guilt that, perhaps, my celebration is tainted, because there is a lot of it that IS secular and that guilt makes me want to respond with a million reasons why it is okay to 'do' Christmas.
Lol, instead I'm responding to you, just to say, I think you pretty well covered why some would respond, even though they weren't asked and their responses may seem more like reasons to have never asked or answered in the first place :p
In my defence, because I do feel like I have to defend celebrating Christmas... I have always found it a beautiful time of year. I know it's rooted in paganism, but I'm not celebrating nature. I'm focusing on the miracle of Jesus' conception and birth. My favorite parts, religiously, are putting together the manger and singing the carols. Otherwise, I get with my family and we bake and we chat and really, it's like we're all kids again, only now we get to play alongside our dcs :) The warmth and sweetness of the season make it so that I couldn't imagine giving it up, nor could I imagine God finding fault in something that brings us so close together. Even if we're not focusing on the death and ressurrection for a month, we are focusing on the the miracles of God and Jesus.