One argument is that some of these women are being forced to dress in this way. It seems a bit odd to me to then want to bring in a law to force them not to wear it. As absent said, for those women whose husbands/families expect them to cover up, this will mean they will be denied access to everyday society.
I don't particularly like to see women dressed in this way but, from what I've seen, it's quite often young women who do this. It could be an expression of loyalty to one's culture and perhaps is also seen as a demonstration of courage. It could be the usual rebelliousness that a lot of teenagers exhibit (many older muslims don't want their young women to adopt this form of dress as it makes both the woman and Asian people in general vulnerable to accusations of extremism and to verbal and physical abuse), it could be attention-seeking, or it could be a genuine response to what a person believes is required of their religion.
At the moment, the number of women who wear the niqab is very small, even in areas that are predominantly muslim. But I feel that the more hysteria there is about it, the more likely it is that young people will adopt this mode of dress as a matter of principle.