The danger with women-only events is that they become perceived as a ghetto. By opening the doors to all, the entire community has a voice, which is as it needs to be if women are to be full, mainstream members of tech communities.
There's a place for "safe spaces" if people are sharing extremely personal stories (e.g. sexual assault, legal issues), but those tend to be done best as small groups anyway - it's a lot harder for a 1000 person auditorium to be a true "safe space".