I suppose the reason they might celebrate christmas on other planets is the same reason all aliens have British accents on Doctor Who or American ones on Star Wars. I hope next year's Doctor Who christmas episode is set on Skaro where I imagine they can feature some great Dalek holiday that will be far more interesting to watch. Perhaps Davros' Birthday is their one day off from exterminating every year, so they must really do it up right.
I have always liked Doctor Who in the past because it has celebrated solving problems by using critical thinking. That is the exact opposite of christianity, and no matter how secular you want to try and make christmas, it still has the word 'christ' in it, so its still a celebration of not thinking for yourself.
Doctor Who has generally treated religion with the skepticism it deserves as this essay illustrates. For a slightly different spin check out The Spirituality of Doctor Who by Phil Purser-Hallard. I will rest my judgment on The Christmas Invasion until after I have seen it of course, but the clip from the show making the chat-show rounds featuring the flying christmas tree has really turned me off. But that's fine, since this is a page about Torchwood not Doctor Who. I will try best I can to ignore the religious-holiday theme and just be watching TCI for clues about our new show starring Captain Jack.
If RTD wants to ruin WHO I suppose he is entitled too since he is the one who resurrected the show, but I would just refer him to a great quote from the Doctor in 1975's The Face of Evil written by Chris Boucher. It illustrates that skepticism has always been part of Doctor Who, and the quote also does a good job of summing up the leaders and followers of religion in a nutshell:
The very powerful and the very stupid have one thing in common, they don't alter their views to fit the facts, they alter the facts to fit their views.