I could never go braless and don't care to see others doing so either, unless you're small enough that its not incredibly noticeable that you are braless, maybe A-B cup, but bigger than that gets questionable IMO.
I don't have a problem with it with small boobed chicks with tiny nipples, and they are in casual clothes (like the pics of Charlize Theron and Kate Moss). I do think it looks ridic in an evening gown, with really pointy nips (that first pic looks horrible) or with giant areola that show through.
I could never go braless because my D cup boobs droop, and that is neither attractive nor comfortable.