Wearing a niqab is done by me seeking Allah's pleasure. If Allah is pleased with me that's what matters to me. People may commit crimes even in a hijab then you ban the hijab too? Disguises are done in various ways and banning the niqab will only be one disguise less!
Lastly if Allah thought wearing a niqab will promote more evil than good then why would He, The All Knowing, recommend it? I believe He alone is the best of judges and I will abide by His laws specially in a country that allows it.