Johnson argues that perpetuating the lie teaches children to believe things without evidence, doggedly stick to your beliefs regardless of the facts, and massage the evidence to support one's own world view.
Now Terry Pratchett espouses quite another view in his book Hogfather. He says that making kids believe the little lie of Santa Claus teaches them to believe in the big lies like Justice and Fairness. At first blush, that's alarmingly cynical. At next blush, though, that's actually kind of genius. We make kids recite the pledge every day knowing full well "justice for all" is a pipe dream. How many little activists are forged in the moment that their belief in universal Justice goes up in flames? If they never believed the lie, they wouldn't be filled with righteous indignation when they found out the truth.
So is Johnson right or is Pratchett? I don't know. The Santa lie has always seemed pretty cruel to me, but then I've always been one to suck the fun out of innocent things.