If you watch only 10 minutes of the video on science parenting from last night's #pubscience, check out the ~30-40min mark. Two important points are made during that span:
1) Babyattachmode makes the under appreciated point about academic career pairs and priority. She talks about how she and her husband got their PhDs around the same time, but because she took time off to have a child, his CV is now stronger. The result of that is that she has essentially become the "trailing spouse" in their job search.
2) Michael Tomasson made one of the more insightful comments about effort. He relayed his experience in which he found that the idea that both parents can shoulder 50% of the load and meet in the middle, is false. Rather, it takes both parents feeling like they are doing 75% for things to come together in anything resembling equal effort. Why 75%? Because there's significant non-overlap between each person's priorities and doing 50% of what you see as 100% leaves significant gaps.
I think both these are worth considering when we talk about dual career pairs, fairness and how kids fit into the mix.