One source of confusion in this thread, I think, is that some people are talking about buying Christmas presents for their pets, while others are talking about buying pets as Christmas presents.
The one, I believe, is a good thing. I know my cats just love it when wrapping paper is being torn off of presents, and thrown around the rooms, and they enjoy being able to play with some of that stuff themselves. A new toys is always nice, although they generally prefer the wrapping paper and the boxes.
The other, buying a pet for someone for Christmas, is probably not a good idea, for all of the reasons stated here. In fact, except for parents getting a pet for their children, it's probably not a good idea to buy a pet for anyone other than yourself. Either way, I agree that Christmas is not the best time to introduce a new pet into the household.