You gain an extra level of deniability if you dont ask. Also its about how its received by the interviewee. If i ask how your summers going and you offer up you have 5 kids, thats different than me asking if you have kids. If i bring up your family situation or kids, the natural question arises, "why would he care about that?" And it can guide the mind to discrimination. People are aware of the fact that having kids have a preception in the corporate world, and if they dont want that to influence the process, theyre not going to offer it up. But just like anything, if someone wants to file a frivolous law suit, theyre going to find a way