What I find most interesting here is when the claim adapts itself in the face of evidence. A friend of mine at 6/7 saw his dad filling the stocking etc. and a group of us decided that the most likely solution was that Santa existed, but delegated certain of his activities to families in some cases, as he had so much on his plate.

More generally, I'm inclined to think it's harmless. If anything, I lean towards the 'helps develop rationality', but that's probably biased because I like Christmas.