When one posts a wishlist and there is more than one person who is likely to get something from that wishlist for that person, there is always the chance of communication not happening and the people getting the same thing. Sometimes it's something that can stand duplication. Sometimes it's awkward.
...If one made a post about holiday wishlist, with the e-mail comments explicitly turned off, and promised not to peek until after the holiday season, then one's friends could discuss what was being gotten without fear of spoilers for the person targeted by gift. (And if the person peeked -- their own fault for spoiling themselves!)