It would seem probable that niqab wearers don't particularly want to talk to you anyway. However, for example, if a niqab wearer goes up to the counter at a library, hands over a library card and a selection of books and says "I would like to check these books out, please", sight of the niqab wearer's face will not assist the librarian in any way, since the message is clear and no library card I've ever seen had a photo on it.