I gave the example of a pollution mask in response to your comment:
Would you accept that pollution-protection does constitute a rational argument for wearing a face-mask, and that therefore your previous 'no rational argument' comment was not correct?
By the way, if I'm cycling in London, I often wear a pollution mask AND wrap-around sunglasses - the latter to protect my eyes from dust/particulates, with uv-protection as an added bonus. Between the mask and the sunglasses, my entire face is pretty much covered. Does that change the argument in your mind?
For what it's worth, I'm undecided on the whole issue of the Burqa/niqab, as I think there are some reasonable arguments on both sides, but I think my own example shows it's a complex issue, because there clearly are legitimate reasons to cover your face in many situations. So simplistic statements along the lines of 'no rational argument for wearing any sort of face mask' don't really help.