Joy, I remember feeling really embarrassed when I found out Santa wasn't real, as well as somewhat angry at my parents for participating in such a longstanding lie. Obviously that was when I was young and I understand the larger context now, but if I had kids I don't think I would perpetuate the Santa thing.
What many of you are talking about is why I find the expansion of Christmas so frustrating. It's not even secularization at this point, it's gotten so everyone no matter what their religion or background is not only allowed to participate, but expected to. When I was a kid you could say "oh no, I'm Jewish" or whatever, and that was enough explanation. Now the commercial and gift giving aspects are so normalized that you can't opt out without being seen as a grinch and a horrible person. And the pre-christian traditions have been rolled up into Christmas, for sure, but it at base is still a Christian holiday celebrating the birth of Christ, and to pretend that it's not and is about 'giving' and 'family' or whatever - I mean, maybe it is for individual people and that's great, but it's disengenous to say it's not Christian at its base, and thus assimilationist, as grad fashionista mentions.
I was raised Christian and my family of origin as well as my husband's is still religious, so I participate in some of the religious and some of the secular stuff when I visit them, and a bit of gift giving and minor decorating at home. I celebrate all the solstices, equinoxes, and cross quarter days already (I'm a bit of a Buddhist pagan fusion at this point), so I usually do a little something for the winter solstice, but by myself, not as part of a larger group.
I honestly don't know what I'd do about Christmas if I had kids. It would be hard for me to not let them participate in some way, because I'm not specifically Jewish or Hindu or even Christian in a way that would make me avoid the secular aspects and celebrate it as a religious occasion only. But I don't know that I'd feel good about letting it be just about presents and Santa, either.