A few decades ago, when I rode a bicycle everwhere, in every kind of weather, I remember going to the liquor store when it was about minus 20, and I had a ski mask on. I was slightly annoyed when I was asked to remove it, but complied with a smile, because I fully understood the reason for removal (Hint: it had something to do with security).
These days, when I see a woman of Middle-Eastern descent wearing a head-scarf (or hijab, as shown below), I get the same feeling as when I see a Jew wearing a yarmulke, a Sikh wearing a turban, or a Christian wearing a cross. Some small part of me rejoices in the possibility that maybe that person is a bit less of a crass materialist than I am. And I'm also reminded that the Muslim, Jew, and Christian are all allied with each other as members of an "Abrahamic religion" (ie, the Old Testament, the New Testament, and the Koran have quite a bit more in common than most people know about, including a veneration of the such prophets as Abraham and Jesus).

Image source: www.channel14.com
However, when I first saw a woman in a niqab dutifully shuffling along behind her husband, a shiver went down my spine, and my eyes spat out daggers at the ignorant oaf who forced his wife to wear such an abomination.
Not long after that, I went to Wikipedia to see if Mohammed's wife would have worn a niqab. It tells us that: "According to the majority of Muslim scholars and Islamic schools of thought, the niqab is not a requirement of Islam." That's good enough for me.
I also remember doing an internet search on this subject a few years ago, and found one source who said that the origin of full face coverings likely came about in societies where the rule of law was absent, and where the strong could simply grab another person's property if the strong wanted that property. And, of course, in some patriarchal societies, women were property, and thus their beauty needed to be somehow hidden from sight. I can't find that source any more, so if anyone knows of it, please email me: Evan Bedford
I find it deeply disturbing that some of the people who most vigorously defend the niqab are professed feminists. What has the world come to?
So let me state the obvious: there is no slippery slope. We, as an advanced Western Society, can state that there is a clear dividing line between the hijab and the niqab. The former is a commendable statement similar to a yarmulke or a cross. The latter is sheer idiocy and a potential security risk to boot.