I'm aware that anyone can hide an envelope of anthrax or some kind of small explosive device under everyday underwear and then post it in a busy place - or rip a gun out of a clip under the coat (raw nuclear devices are more problematic because stuff like bulk enriched uranium emits heat that makes it impossible to just slip under the dress or the briefs and walk throiugh a crowd without notice). I never said you could make your country fail safe against terror acts by forbidding burqas in public places, of course one can't. But if we're talking of groups and not just isolated lunatics, it's harder for a terrorist group of any kind to keep up a drive of assaults if they have to make their carriers show their faces and leave clear images of how they looked on the scene, who they were, images and data that might be used by the police. With a garment that hides the face and much of what's distinctive about the body, and which is mostly used by women - not the kind of armed militants people will expect in most places - it becomes more inviting to equip women with bombs and send them into crowds. Whether they die in the attack or they escape, they fell they can act under a shield of "I'm not going to be tracked" and if you remove that feeling of secure anonymity, recruitment for the militant groups becomes harder too. No society can protect itself 100% against isolated madmen but most political killers are not completely isolated.