You realise that by saying "It's a slippery slope what you allow and don't allow" you are perfectly validating one of the major reasons people say it shouldn't be banned....
And as for hiding a knife in a hijab, that's an absolute nonsense reason... unless you also think we should ban trenchcoats, suit jackets, non skin tight jeans, backpacks, suitcases, sweaters etc etc etc.
There are 100 places to hide a knife on a person, none of which specificly require the clothing of any religion.